This document discusses infection, including its classification, sources, modes of transmission, and factors that influence pathogenicity. It defines saprophytes, commensals, and pathogens. Infections can be primary, secondary, local, cross, or nosocomial. Sources of infection include humans, animals, insects, soil, water, and food. Pathogens can be transmitted via contact, inhalation, ingestion, inoculation, insects, congenitally, or iatrogenically. Factors like transmissibility, adhesion, toxins, and host defenses influence a microbe's ability to cause disease.

1. INFECTION

2. CONTENTS
• Introduction
• Classification of infection
• Sources of infection
• Modes of transmission of infection
• Factors predisposing to microbial Pathogenicity

3. INTRODUCTION
Saprophytes :are free-living microbes that live on dead
or decaying organic matter
Commensal: :normal bacterial flora of the body
Pathogen :can establish themselves and multiply in the
hosts
1. Primary pathogen
2. Opportunistic pathogen
• The lodgment and multiplication of a parasite in or
on the tissues of a host called infection.

4. CLASSIFICATION OF INFECTIONS
Primary infection: Initial infection
Reinfection: Subsequent infections by the same parasite
Secondary infection: new parasite infection in a host whose
resistance is lowered
Local infection: sepsis at localized sites such as appendix or
tonsils, generalized effects are produced.
Cross infection: When in a patient already suffering from a
disease a new infection is set up from another host
Nosocomial infections: Cross infections occurring in hospitals
are called nosocomial infections.

5. • Iatrogenic infection: physician induced infections resulting
from investigative, therapeutic or other procedures.
• Inapparent infection: clinical effects are not apparent.
• Atypical infection: the typical or characteristic clinical
manifestations of the particular infectious disease are not
present.
• Latent infection: Some parasites may remain in the tissues in
a latent or hidden form proliferating and producing clinical
disease when the host resistance is lowered.

6. SOURCES OF INFECTION
1. Human beings
2. Animals
3. Insects
4. Soil and water
5. Food

7. • A carrier is person who harbors the microorganisms
without suffering from any ill effect, because of it.
1. Convalescent carrier:
2. Healthy carrier:
3. Incubatory carrier:
4. Temporary carriers:
6. Contact carriers:
7. Paradoxical carrier:

8. ANIMALS
Zoonosis : diseases transmissible to man from animals
Bacterial :Anthrax, brucellosis, Q fever, leptospirosis, bovine
tuber-culosis, bubonic plague, Salmonella food poisoning.
Viral : Rabies, yellow fever, cowpox, monkeypox
Parasitic: Leishmaniasis, toxoplasmosis
Helminthic Echinococcosis, teniasis, trichinellosis
Fungal : Microsporum canis, Trichophyton verrucosum.

10. INSECTS
• Blood-sucking insects such as mosquitoes, ticks,
mites, flies, and lice may transmit pathogens to
human beings and diseases so caused are called
arthropod borne diseases.
• Insects that transmit infections are called vectors
 Mechanical vector:
 Biological vectors:

12. SOIL, WATER AND FOOD
Soil
a. Spores of tetanus and gas gangrene:
b. Fungi and parasites:
Water :
Shigella, Salmonella, Vibrio cholerae, poliomyelitis virus,
hepatitis virus) or due the presence of aquatic vector (cyclops
containing larvae of guinea worm infection.
Food
• food poisoning, gastroenteritis, diarrhea and dysentery. There
are two primary types of food-related diseases: foodborne
infections and food intoxicants.

13. MODES OF TRANSMISSION OF INFECTION
1. Contact
2. Inhalation
3. Ingestion
4. Inoculation
5. Insects
6. Congenital
7. Iatrogenic and laboratory infections

14. CONTACT
• Direct contact: STD, leprosy, leptospirosis,
skin and eye infections.
• Indirect contact: Fomites

16. Inhalation and Ingestion
• Inhalation:
• Respiratory infections such as common cold,
influenza, measles, mumps, tuberculosis and
whooping cough are acquired by inhalation.
• Ingestion:
• acquired by the ingestion of food or drink
contaminated by pathogens e.g. acute
diarrheas, typhoid fever, cholera, polio,
hepatitis

19. • Inoculation:
• The disease agent may be inoculated directly
into the skin or mucosa, e.g. rabies virus
deposited subcutaneously by dog bite, tetanus
spores implanted in deep wounds, and
arboviruses injected by insect vectors.
• Congenital :
• Some pathogens are able to cross the placental
barrier and reach the fetus in utero.

22. FACTORS PREDISPOSING TO MICROBIAL PATHOGENICITY
• Pathogenicity is the ability of a microbial species to
cause disease.
• The term virulence denotes the ability of a strain of
a species to produce disease.

24. Determinants of Virulence
1. Transmissibility
2. Adhesion
3. Invasiveness
4. Toxigenicity
5. Avoidance of host defence mechanisms
6. Enzymes
7. Plasmids
8. Bacteriophages
9. Communicability
10. Infecting dose
11. Route of infection