Microbial pathogenicity is the ability of microorganisms to cause disease, involving complex interactions with the host's immune system and tissues. Pathogens like bacteria, viruses, fungi, and parasites utilize various mechanisms to invade and colonize host tissues, evade immune responses, and produce virulence factors, which enhance their ability to cause infections. Understanding these processes is crucial for developing effective treatments and preventive measures against infectious diseases.

1. Dr Sumitha J
Associate Professor
Department of Microbiology
JBAS College for Women

2. Microbial pathogenicity refers to the ability of
microorganisms, such as bacteria, viruses, fungi, and
parasites, to cause disease in their host organisms. It
involves a complex interplay between the microorganism and
the host's immune system, tissues, and physiological
processes. Pathogenic microorganisms have evolved
various mechanisms to successfully infect, colonize, and
replicate within their host, leading to a range of diseases with
varying degrees of severity.
Microbial Pathogenicity
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3. It's important to note that not all microorganisms are pathogenic.
Many microorganisms are part of the normal human microbiota and
contribute to health by competing with potential pathogens and
supporting various physiological processes. Pathogenicity is a
complex trait influenced by the interplay between microbial factors
and host factors such as genetics, immune status, and overall
health.
Understanding microbial pathogenicity is crucial for developing
effective treatments, preventive measures (including vaccines), and
strategies to control and manage infectious diseases.
Microbial Pathogenicity
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4. Key factors

5. Host Invasion: Pathogenic microorganisms must be able to enter
the host's body or specific tissues. This can occur through inhalation,
ingestion, direct contact, or other means.
Microorganisms, including bacteria, viruses, fungi, and parasites,
have evolved diverse strategies to invade host organisms and initiate
infections. Their ability to successfully invade and colonize the host is
a critical step in the development of infectious diseases. Here's a
closer look at how each group of microorganisms achieves host
invasion
Key Factors
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6. Bacterial invasion involves several steps that allow bacteria to breach host barriers and
establish infection:
• Adhesion: Bacteria possess adhesins on their surfaces that enable them to bind to
specific receptors on host cells or tissues. This attachment is essential for colonization.
• Colonization: Once attached, bacteria begin to multiply and form colonies. Biofilm
formation can provide bacterial communities with protection and enhance their
persistence.
• Tissue Penetration: Some bacteria produce enzymes that break down host tissues,
facilitating their penetration into deeper layers. For example, Streptococcus pyogenes
secretes enzymes that aid in tissue invasion.
• Invasion of Immune Cells: Certain bacteria, like Salmonella and Mycobacterium
tuberculosis, are phagocytosed by immune cells but can resist destruction and replicate
within these cells.
Host Invasion-Bacteria
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7. Viruses:
Viral invasion involves the following steps, typically at the cellular level:
• Attachment and Entry: Viruses have specific attachment proteins that recognize
receptors on host cells. Once attached, viruses can enter the host cell through
fusion, endocytosis, or other mechanisms.
• Release of Genetic Material: Once inside the cell, viruses release their genetic
material (DNA or RNA) and take control of the host's cellular machinery to replicate.
• Replication and Assembly: Viral genetic material is replicated, and new viral
components are synthesized within the host cell. These components are assembled
into new virus particles.
• Release: New virus particles are released from the host cell, often causing cell
lysis. These particles can go on to infect neighboring cells.
Host Invasion-Viruses
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8. Fungal invasion involves complex interactions between fungi and host tissues:
• Adhesion: Fungi use adhesins and other surface molecules to attach to host
tissues. Candida species, for instance, adhere to mucosal surfaces in the oral and
genital tracts.
• Hyphal Growth and Penetration: Invasive fungi like Aspergillus and Candida can
extend hyphae (filamentous structures) that penetrate host tissues, often exploiting
areas of damage or immune suppression.
• Tissue Invasion: Fungi secrete enzymes that degrade host tissues, facilitating their
invasion. They can also induce inflammation, promoting tissue destruction.
• Systemic Spread: Some fungi can enter the bloodstream and spread to different
organs, causing systemic infections. The ability to switch between yeast and hyphal
forms contributes to their invasiveness.
Host Invasion-Fungi
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9. Parasites:
Parasitic invasion involves multiple steps tailored to the specific type of parasite:
• Attachment: Parasites use various structures like hooks, suckers, or adhesins to attach to host
tissues. For example, hookworms attach to the intestinal lining.
• Penetration: Parasites can actively penetrate host tissues or enter through natural openings. For
instance, protozoan parasites like Entamoeba histolytica can directly invade the intestinal wall.
• Migration: Some parasites, such as helminths (worms), migrate through host tissues to reach their
preferred sites. This migration can cause tissue damage and inflammation.
• Localization and Reproduction: Once in their preferred location, parasites establish themselves and
reproduce. Malarial parasites, for instance, reproduce within red blood cells.
• Immune Evasion: Parasites often employ mechanisms to evade immune responses, such as altering
surface antigens to avoid detection.
Understanding the mechanisms by which bacteria, viruses, fungi, and parasites invade their hosts is
crucial for developing effective strategies to prevent, diagnose, and treat infectious diseases.
Host Invasion-Parasite
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10. Adherence and Colonization: Once inside
the host, pathogens often need to adhere to
host cells or tissues to establish a foothold.
Adhesion is often facilitated by specialized
surface structures like adhesins.
2.Adherence & colonisation
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11. • Adhesion Mechanisms: Bacteria possess adhesins—surface
molecules, often proteins or glycoproteins—that specifically bind to host
receptors. These receptors can be found on host cells, extracellular
matrices, or other surfaces.
• Specificity: Different bacterial species have adhesins that recognize
different host molecules, ensuring specificity in the attachment process.
• Biofilm Formation: Some bacteria form biofilms, complex
communities of bacteria encased in a protective matrix. Biofilms allow
bacteria to adhere to surfaces and protect themselves from immune
responses and antibiotics.
• Colonization and Multiplication: Once attached, bacteria multiply
and form colonies. Biofilm-associated bacteria are often more resistant
to the host immune system and antimicrobial treatments.
Adherence & colonisation -Bacteria
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12. • Attachment Proteins: Viruses use attachment proteins, typically located on
their outer surfaces, to recognize specific receptors on host cells. These
receptors can be proteins, glycoproteins, or carbohydrates.
• Binding Specificity: Viruses exhibit high specificity in their attachment due to
the interaction between viral attachment proteins and host cell receptors.
• Endocytosis and Fusion: After attachment, viruses enter host cells through
receptor-mediated endocytosis or membrane fusion, depending on the virus
type.
• Intracellular Replication: Once inside the host cell, viruses utilize host cellular
machinery to replicate their genetic material and produce new virus particles.
Adherence & colonisation -Viruses
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13. • Adhesion Structures: Fungi employ various adhesion structures, including
adhesins and surface proteins, to adhere to host tissues. Candida species, for
example, have adhesins that interact with host cells.
• Hyphal Penetration: Invasive fungi like Aspergillus and Candida extend hyphal
structures that penetrate host tissues. Adhesion molecules facilitate this process.
• Tissue Binding and Invasion: Fungi can secrete enzymes that degrade host
tissues, creating entry points. Adhesion molecules then help establish a strong
attachment, aiding in tissue invasion.
• Biofilm Formation: Similar to bacteria, certain fungi can form biofilms,
particularly in medical devices like catheters. Biofilms provide protection and
enhance colonization.
Adherence & colonisation -Fungi
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14. • Attachment Structures: Parasites utilize various attachment structures such
as hooks, suckers, and adhesins to adhere to host tissues. For example,
hookworms use specialized mouthparts to attach to the intestinal lining.
• Specific Receptors: Parasitic attachment is often mediated by interactions
between parasite ligands and host receptors.
• Penetration and Migration: After attachment, some parasites actively
penetrate host tissues and migrate to their preferred sites of colonization.
• Tissue Adhesion and Establishment: Adhesion mechanisms are essential
for the establishment of parasitic infections. Once attached, parasites can
avoid host immune responses and begin reproduction.
Adherence & colonisation -Parasite
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15. Microorganisms, including bacteria, viruses, fungi, and parasites, have
evolved intricate strategies to evade the host's immune responses. This
ability to circumvent the immune system allows these pathogens to
establish infections and persist within their hosts. Here's an overview of
how each group of microorganisms evades immune responses
Understanding how microorganisms evade immune responses is
essential for designing effective treatments, vaccines, and interventions
to combat infectious diseases. These strategies highlight the complex
arms race between pathogens and the host immune system.
3.Evading Immune Response
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16. • Antigenic Variation: Some bacteria can change the antigens displayed on their surfaces,
rendering the host immune system unable to recognize them effectively. This is seen in
pathogens like Neisseria gonorrhoeae and Borrelia burgdorferi (causing Lyme disease).
• Capsule Formation: Bacteria can produce capsules that prevent immune cells from
engulfing them (phagocytosis). For instance, Streptococcus pneumoniae's capsule helps it
evade immune detection.
• Mimicking Host Molecules: Bacteria can produce molecules that mimic host antigens,
confusing the immune system and hindering an effective response.
• Inhibiting Complement Activation: Some bacteria inhibit the activation of the complement
system, a crucial part of the immune response that aids in pathogen destruction.
• Suppressing Immune Cells: Bacteria can produce substances that suppress immune cell
function, reducing the host's ability to mount an effective defense.
Evading Immune Response-Bacteria
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17. • Antigenic Drift and Shift: Viruses, especially RNA viruses like influenza, undergo frequent
mutations (antigenic drift) or reassortment (antigenic shift), leading to changes in surface
antigens. This makes it challenging for the immune system to recognize them.
• Latency: Some viruses establish latent infections, where they remain dormant in host cells
without triggering a strong immune response. Herpesviruses are known for this strategy.
• Immunoevasion Proteins: Viruses can produce proteins that interfere with host immune
pathways. For instance, HIV produces proteins that impair immune cell function.
• Downregulating Immune Molecules: Viruses can downregulate the expression of major
histocompatibility complex (MHC) molecules on infected cells, reducing their recognition by
immune cells.
• Blocking Interferon Response: Interferons are signaling proteins that play a role in antiviral
defenses. Some viruses produce proteins that inhibit interferon responses.
Evading Immune Response-Virus
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18. • Surface Variation: Similar to bacteria, fungi can alter surface antigens to avoid
immune recognition. Candida species can switch between different forms, evading
immune responses.
• Capsule Production: Some pathogenic fungi produce capsules that hinder
phagocytosis and immune cell recognition. Cryptococcus neoformans is known for its
capsule-mediated immune evasion.
• Modulating Immune Cell Function: Fungi can produce substances that suppress
immune cell activity or alter the immune response to favor fungal survival.
• Mimicking Host Molecules: Fungi can produce molecules that mimic host
components, reducing immune cell recognition.
• Antioxidant Production: Fungi can produce antioxidants that counteract host
immune mechanisms, allowing them to survive within immune cells.
Evading Immune Response-Fungi
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19. • Antigenic Variation: Similar to bacteria and viruses, some parasites can change
their surface antigens to evade immune recognition. African trypanosomes are
known for their antigenic variation.
• Modulating Host Immune Responses: Parasites can secrete molecules that
manipulate host immune responses, promoting conditions favorable for their survival.
• Immune Cell Inhibition: Parasites can produce molecules that inhibit immune cell
function or induce immune cell death.
• Suppressing Cytokine Responses: Parasites can dampen cytokine responses,
disrupting communication between immune cells and impairing immune defense.
• Intracellular Survival: Some parasites can enter host cells and survive within
them, avoiding direct exposure to immune surveillance.
Evading Immune Response-Parasite
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20. Virulence factors are specific molecules or mechanisms that
microorganisms employ to enhance their ability to cause
disease and establish infections within their hosts. These
factors contribute to the microorganisms' pathogenicity and
virulence.
Understanding the production of virulence factors by
microorganisms is essential for developing strategies to target
and neutralize these factors, ultimately reducing the severity of
infections and improving treatment outcomes
4.Production of Virulence Factors
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21. • Toxins: Bacteria can produce various types of toxins that cause damage to host tissues
and disrupt normal physiological processes. Examples include:
• Exotoxins: Secreted proteins that can target specific host cells or systems. Examples
include diphtheria toxin and tetanus toxin.
• Endotoxins: Lipopolysaccharides (LPS) found in the outer membranes of Gram-negative
bacteria. They can trigger excessive inflammatory responses in the host.
•Adhesins: Surface molecules that enable bacteria to adhere to host cells or tissues,
facilitating colonization and invasion.
• Secretion Systems: Some bacteria possess specialized secretion systems that allow them
to inject toxins or other molecules directly into host cells. Type III secretion systems are an
example.
• Capsules: Polysaccharide capsules protect bacteria from phagocytosis and immune
recognition, enhancing their survival within the host.
• Enzymes: Bacteria can produce enzymes that degrade host tissues, aiding in tissue
invasion. Examples include proteases and hyaluronidases.
Production of Virulence Factors-Bacteria
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22. • Attachment Proteins: Viruses have attachment proteins on their surfaces that
recognize host cell receptors, facilitating viral entry into host cells.
• Envelope Proteins: Many enveloped viruses have surface glycoproteins that play a
role in attachment and entry. These proteins can be targets for neutralizing antibodies.
• Antigenic Variation: Some viruses undergo frequent genetic changes in surface
proteins, making it difficult for the host immune system to recognize and neutralize them.
• Immunoevasion Proteins: Viruses can produce proteins that interfere with host
immune responses. For example, HIV produces proteins that inhibit immune cell
functions.
• Viral Proteases: Viruses often encode proteases that process viral precursor proteins
into functional components, contributing to viral assembly and replication.
Production of Virulence Factors-Virus
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23. • Adhesins: Fungi possess adhesins that help them attach to host tissues,
promoting colonization and invasion.
• Capsules: Similar to bacteria, some fungi produce capsules that protect
them from immune recognition and facilitate tissue invasion.
• Enzymes: Fungi can secrete enzymes that break down host tissues, aiding
in tissue penetration and invasion. These enzymes include proteases and
lipases.
• Toxins: Certain pathogenic fungi produce toxins that damage host cells and
tissues, contributing to disease progression.
• Morphological Changes: Dimorphic fungi can switch between yeast and
hyphal forms, which can impact their virulence by altering their interaction with
host cells.
Production of Virulence Factors-Fungi
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24. • Surface Molecules: Parasites can express molecules on their surfaces
that aid in adhesion to host cells and tissues.
• Antigenic Variation: Similar to bacteria and viruses, some parasites
change their surface antigens to evade host immune recognition.
• Toxins and Enzymes: Parasites can produce toxins and enzymes that
damage host tissues, promote invasion, and create a favorable
environment for their growth.
• Immune Evasion Molecules: Parasites can produce molecules that
interfere with host immune responses, allowing them to avoid immune
detection and destruction.
• Migration Mechanisms: Some parasites produce enzymes that facilitate
their migration through host tissues, aiding in tissue invasion.
Production of Virulence Factors-parasite
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25. Pathogens can invade deeper tissues, causing direct damage or interfering with
host cellular processes. This can lead to inflammation, tissue destruction, and
disease symptoms Microorganisms employ various mechanisms to invade host
tissues and cause damage, leading to the progression of infectious diseases.
Tissue invasion involves the penetration of microorganisms into host cells,
tissues, or organs, often resulting in localized or systemic damage.
Microorganisms' abilities to invade host tissues and cause damage are central to
their pathogenicity. Understanding these processes is essential for developing
targeted interventions to limit tissue invasion, minimize damage, and mitigate the
impact of infectious diseases.
Tissue Invasion and Damage
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26. • Direct Invasion: Pathogenic bacteria can directly invade host tissues through
mechanisms such as penetration, active migration, or exploiting existing openings.
• Enzymatic Degradation: Many bacteria produce enzymes, like proteases and
collagenases, that break down host tissues, allowing them to penetrate and colonize
deeper layers.
• Inflammation: Bacteria trigger immune responses, resulting in inflammation. While
inflammation is a protective response, it can also contribute to tissue damage if
excessive.
• Toxin-Mediated Damage: Bacterial toxins can damage host cells and tissues directly,
leading to tissue destruction and impairing normal physiological functions.
• Formation of Abscesses: Some bacteria can cause the formation of localized
collections of pus (abscesses), where immune cells and bacteria accumulate, leading to
tissue necrosis.
Tissue Invasion and Damage-Bacteria
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27. • Cell Lysis: Many viruses cause host cells to rupture as they replicate
and assemble new virus particles. This can lead to tissue damage and
cell death.
• Inflammation: Viral infections often trigger immune responses, leading
to inflammation. The immune response itself can contribute to tissue
damage.
• Cytopathic Effects: Some viruses induce specific changes in infected
cells, leading to their death or altered function. For instance, hepatitis
viruses damage liver cells.
• Immune-Mediated Damage: In some cases, the immune system's
efforts to control viral infections can inadvertently cause tissue damage
due to excessive immune responses.
Tissue Invasion and Damage:-Virus
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28. Tissue Invasion and DamageFungi
• Invasion of Host Tissues: Invasive fungi like Candida and Aspergillus
can penetrate host tissues using hyphae or other invasive structures.
• Secreted Enzymes: Fungi secrete enzymes that degrade host tissues,
facilitating invasion. These enzymes can also cause damage to blood
vessels, leading to bleeding.
• Inflammation: Fungal infections often induce inflammation, which can
contribute to tissue damage and destruction of infected areas.
• Formation of Granulomas: In response to some fungal infections, the
immune system forms granulomas, aggregates of immune cells that can
cause tissue damage.

29. Tissue Invasion and Damage-Parasite
• Cell Invasion: Intracellular parasites can enter and replicate within
host cells, leading to cell destruction and tissue damage.
• Tissue Migration: Parasites like helminths (worms) can actively
migrate through host tissues, causing mechanical damage and
inflammation.
• Inflammation and Immune Responses: Immune responses to
parasitic infections often involve inflammation, which can lead to tissue
damage and disruption of normal tissue function.
• Formation of Lesions: Some parasites cause the formation of
lesions in host tissues due to immune responses, inflammatory
reactions, or direct damage caused by the parasites themselves.

30. Systemic Spread of Microorganisms
• The ability of microorganisms to spread from
their initial site of infection to other parts of
the body is a critical factor in the
development and severity of infectious
diseases. Systemic spread can lead to more
severe illness and complications

31. Systemic Spread of Microorganisms: Bacteria
• Bloodstream Dissemination: Some bacteria can enter the
bloodstream (bacteremia) and spread to distant organs and tissues.
Bacteria like Staphylococcus aureus and Streptococcus pyogenes
can cause sepsis when they disseminate systemically.
• Lymphatic System: Bacteria can also enter the lymphatic
vessels and travel to lymph nodes and other sites. This can lead to
the formation of abscesses or localized infections.
• Colonization of Organs: Once in the bloodstream, bacteria can
colonize various organs, causing conditions like endocarditis (heart),
osteomyelitis (bones), or meningitis (brain and spinal cord).

32. Systemic Spread of Microorganisms: virus
• Hematogenous Spread: Many viruses can spread through the
bloodstream to different organs and tissues. Examples include HIV, which can
target immune cells throughout the body, and hepatitis viruses that infect the
liver.
• Neural Spread: Certain viruses can travel along nerve pathways, leading
to the spread to adjacent tissues or even the central nervous system (CNS).
Herpesviruses are known for this mode of spread.
• Respiratory Spread: Respiratory viruses like influenza can disseminate
to distant organs after initial infection of the respiratory tract.

33. Systemic Spread of Microorganisms: fungi
• Hematogenous Dissemination: Invasive fungal infections can
spread through the bloodstream, leading to systemic infections
affecting multiple organs. Candida and Aspergillus species are
examples.
• Lymphatic Spread: Fungi can also disseminate through the
lymphatic system, leading to infections in lymph nodes and adjacent
tissues.
• Respiratory Spread: Some fungal spores can be inhaled,
leading to lung infections, and may subsequently disseminate to
other organs, especially in immunocompromised individuals.

34. Systemic Spread of Microorganisms: parasite
• Hematogenous Dissemination: Bloodborne parasites can spread
through the bloodstream, leading to infections in multiple organs. For
example, Plasmodium species causing malaria can infect red blood cells
and cause organ damage.
• Lymphatic and Neural Spread: Parasites can also disseminate
through the lymphatic system or follow neural pathways, spreading to
different organs or the CNS.
• Migration through Tissues: Helminths (worms) can migrate through
host tissues, causing damage along their paths and potentially reaching
other organs.

35. Transmission of Microorganisms
• The transmission of microorganisms from one host to
another is a critical aspect of the spread of infectious
diseases. Different types of microorganisms have evolved
diverse strategies to facilitate their transmission to new
hosts
• Understanding the modes of transmission is essential for
implementing preventive measures, such as practicing good
hygiene, using barrier methods, avoiding exposure to
vectors, and ensuring proper food and water hygiene.

36. Transmission of bacteria
• Direct Contact: Bacterial infections can be transmitted through direct
physical contact with infected individuals. Skin-to-skin contact, sexual contact, and
contact with bodily fluids are common modes of transmission.
• Respiratory Droplets: Bacteria can be spread through respiratory droplets
produced by coughing, sneezing, or talking. This is a common route for respiratory
infections like tuberculosis and streptococcal infections.
• Fecal-Oral Route: Some bacterial infections are transmitted through
contaminated food, water, or surfaces, where oral ingestion of the pathogen
occurs.
• Vector-Borne Transmission: Certain bacteria are transmitted by vectors such
as ticks (Lyme disease), fleas (plague), and mosquitoes (dengue, malaria).

37. Transmission of virus
• Respiratory Droplets: Many viral infections, including the common cold
and influenza, are transmitted through respiratory droplets expelled during
coughing and sneezing.
• Direct Contact: Skin-to-skin contact or contact with contaminated
surfaces can transfer viruses from one person to another, especially in the
case of skin infections like herpes.
• Fecal-Oral Route: Viral infections such as norovirus and hepatitis A can
be spread through ingestion of contaminated food, water, or objects.
• Vector-Borne Transmission: Some viruses are transmitted by arthropod
vectors like mosquitoes (dengue, Zika, West Nile) or ticks (Lyme disease).

38. Transmission of Fungi
• Direct Contact: Some fungal infections are transmitted through direct
contact with infected individuals, often involving skin-to-skin contact.
• Airborne Transmission: Fungal spores can become airborne and be
inhaled by individuals, leading to respiratory infections. This is seen in fungal
infections like aspergillosis.
• Environmental Exposure: Exposure to environments with a high fungal
load, such as certain soil types, can lead to fungal infections.
• Contaminated Objects: Fungal spores can survive on surfaces, leading
to indirect transmission through contact with contaminated objects.

39. Transmission of Parasite
• Vector-Borne Transmission: Many parasites are transmitted through
vectors like mosquitoes (malaria, dengue), ticks (Lyme disease), and flies
(African trypanosomiasis).
• Fecal-Oral Route: Some parasites, such as protozoa causing
amoebiasis, can be transmitted through ingestion of contaminated food or
water.
• Direct Contact: Skin-to-skin contact or contact with contaminated
surfaces can lead to transmission of parasites like scabies mites.
• Ingestion of Intermediate Hosts: Some parasitic infections involve the
consumption of undercooked or raw intermediate hosts, such as fish or pork,
which contain parasitic larvae.