INTRODUCTION TO INFECTIOUS DISEASES

1. Introduction to Infectious
Diseases
Lecture 1

2. Learning Objectives
Infectious Diseases
1. Define infectious diseases
2. Enlist common infectious agents and diseases related to them
3. Identify mode of infection/spread of various microbial agents
4. Define pyemia, bacteremia, toxaemia, septicaemia and
subclinical infection.
5. Emerging and re-emerging infectious diseases

3. Definitions
Infectious Diseases
Caused by infectious agents
Infectious Agents
Microorganisms; viruses, bacteria , fungi and parasites
Infection
An uncontrolled growth of harmful microorganisms in a host causing tissue damage
Super added Infection or super infection
A second infection superimposed on an earlier one, especially by a different
microbial agent of exogenous or endogenous origin, that is resistant to the treatment
being used against the first infection

4. Definitions
Communicable Diseases transmitted from one host to another
Contagious Diseases spread from person to person
Subclinical Infection without significant sign and symptoms
Opportunistic Infections caused by non-pathogenic low virulence
microorganisms in immunocompromised
individuals

6. Classification of Infectious Agents
Bacteria (cocci &bacilli)
Gram-negative or Gram-positive
Viruses
DNA virus or RNA virus
Enveloped vs non-enveloped
Fungi
Disseminated or Localized
Parasites
Protozoa
Helminths
Prion (protein aggregates, lack nucleic
acid)
Viroid composed of single-stranded
RNA without protein coat
Plasmid is a extrachromosomal double
stranded small DNA molecule naturally
exist in bacterial cells
Microorganisms, ranging from less than
20nm (prion protein aggregates) to
viruses, bacteria , fungi and parasites
upto 10-meters in length e.g tapeworms

8. Definitions
Bacteremia Presence of small number of low virulence viable bacteria in
the bloodstream which do not multiply significantly e.g S. typhi, E.coli
Septicemia Presence of rapidly multiplying highly pathogenic bacteria (e.g
Pyogenic Steph.cocci) in the bloodstream.
Septicemia can lead to;
- Sepsis
- Pyaemia
- Toxemia

10. Septecemia

11. Definitions
Sepsis; A whole-body inflammatory state leading to multiple organ failure and
death resulting from sepsis-induced hypotension and diffuse intravascular
coagulation (DIC)
Pyaemia; the organisms escape into blood stream in the form of small
aggregates (micro-emboli) results in pyaemic abscesses or septic infarct, due to
septic thrombosis of larger veins in various organs e.g lung, liver brain , kidney
and heart
Toxemia; blood poisoning by toxins from a local bacterial infection

12. Definitions
Viremia
Circulating viruses in the blood stream
Latent virus infection
Reactivation of infection from virus in latent phase
e.g. Herpes zoster infection in childhood with vesicular skin rash get
resolved, many years later reactivation of infection from the virus DNA which
lies in latent phase in dorsal root ganglia
Oncogenic virus infection virus which can give rise to tumor e.g. HPV,
HBV, HCV, EBV, HTLV

13. Host Barriers Against Infectious Agents
Skin: Constantly sloughing keratin layer and normal skin flora
Respiratory system: Alveolar macrophages and muco-ciliary clearance by
bronchial epithelium
GI system: Acidic gastric pH, mucus, secretions, pancreatic enzymes, bile,
defensins, IgA, and normal gut flora
Urogenital tract: Repeated flushing and commensal flora

14. Routes of Transmission of Infectious Agents
1. Inhalation
2. Ingestion
3. Contact
4. Sexual transmission
5. Insect
6. Animal bites
7. Injection
8. Vertical transmission

15. Routes of entry, dissemination, and release of microbes from the body

16. Phases of Infectious Disease
Incubation period; time between infection and the appearance of signs and
symptoms
Prodromal phase; mild, nonspecific symptoms that signal onset of some
diseases
Clinical phase; a person experiences typical signs and symptoms of
disease
Recovery phase; symptoms have disappeared, tissues heal, and the body
regains strength

17. Emerging Infectious Disease
Newly identified and previously unknown infectious agents that cause public
health problems either locally or internationally e.g in 20th centaury (HBV, HCV)
and in 21st centaury (SARS, MERS, Bird Flu, Swine flu)
Re-emerging Infectious Disease
Infectious agents known for some time, had fallen to low levels, that no
longer considered public health problems and are now showing increased
incidence or prevalence worldwide e.g Cholera, Dengue fever, Zika virus
and in Bioterrorism ( B. anthracis, C. botulinum toxin)

18. • Disease control
• Disease prevention
• Disease elimination
• Disease eradication

19. Assignment
Q. Why it is important to have the knowledge of infectious
diseases?

20. Learning Outcomes
1. Recognise infectious diseases, pyemia, bacteremia,
toxaemia, septicemia and subclinical infection.
2. Determine different modes of spread of microbial
agents.
3. Identify Emerging and Remerging infectious
diseases.

21. References
• Reading : Robbins Basic Pathology, 10th Edition
(2017), By: Kumar, Abbas, Aster
• Web Path
• WWW.fleshandbone.com
• WWW.studentconsult.com