Class notes Biomedical Sciences (BSc) Medical Microbiology, ISBN: 9780323086929

BB2716 Medical Microbiology
Introduction / Infectious disease epidemiology

- Definitions
o Microbiology: The study of organisms
o Microorganisms/microbes: Microscopic organisms
o Pathogens: Susceptible to cause of disease
o Infectious disease: Disease caused by an infectious agent, a pathogen
o Medical microbiology: The study of microorganisms that are of medical
importance and susceptible of causing disease in
human beings
- Microorganisms can be found in every ecosystem. They populate the heathy human body by
billions. Some of them participate in bodily functions, e.g. bacteria in the intestinal tract.
Only a few species of microorganisms are harmful to humans an can cause disease by:
o Producing toxic compounds
o Direct infection
- Disease is a disturbance in the state of health. It is when the illness is clinically evident by
characteristic medical signs and symptoms. Microbes cause disease in the course of stealing
space nutrients, and/or living tissue from their hosts (e.g. humans).
- To cause disease, microbes must be able to:
o Gain access to the host (contamination)
o Adhere to the host (adherence), meaning sticking to the host
o Replicate on the host (colonisation)
o Invade tissues (invasion)
o Harm the host: production of toxins, alteration of host functions, destruction of host
tissues (damage)
- Examples of microbes/infectious agents:
o Prokaryotes
o Fungi
o Protozoa
o Helminths
o Viruses
o Prions
- Difference between epidemiology and infectious disease epidemiology:
o Epidemiology
 Deals with one population
 Identifies cause
 The risk is the cause
o Infectious epidemiology
 Deals with two or more populations
 The cause is unknown
 The case is a risk factor
- The disease can be carried out by infectious agents, vectors (such as mosquitoes, snails or
blackflies that carry the infectious agents) or animals
- The cause is often unknown, but we know that the infectious agent is a necessary cause.
- The case is often the risk factor, meaning that infection in one person can be transmitted to
others

,- Infectious disease epidemiology is used for:
o Identification of cause of new, emerging infections, e.g. HIV, SARS
o Surveillance of infectious disease
o Identification of source of outbreaks
o Studies of routes of transmission and natural history of infections
o Identification of new interventions
- Infectious disease: Disease caused by an infectious agent
- Communicable disease: Disease caused by transmission, either directly or indirectly
- Non-communicable disease: Disease caused by outside hosts or opportunistic pathogens
- Transmissible disease: Disease caused by transmission through unnatural routes
- Subacute disease: Disease with a time course and symptoms between acute
and chronic
- Asymptomatic disease: Disease without symptoms
- Contagious disease: Communicable disease that is easily spread
- Systemic disease: Widespread infection in many systems of the body; often
travels in the blood or lymph
- Focal disease: Infection that serves as a source of pathogens for infections
at others site in the body
- Latent disease: Disease that appears a long time after infection
- Routes of transmission:
o Direct o Indirect
 Skin-to-skin (Herpes type )  Food-borne (Salmonella)
 Mucous-to-mucous (STI)  Water-borne (Hepatitis A)
 Across placenta (Toxoplasmosis)  Vector-borne (Malaria)
 Through breast milk (HIV)  Air-borne (Chickenpox)
 Sneeze-cough (Influenza)  Ting-borne (Scarlatina/Scarlet
fever)
- Exposure to infectious agents: there should be a relevant contact – skin, sexual intercourse,
water contact etc. - depending on the infectious agent.




- There are two timelines for infection
o Susceptible → Infection → Latent period →Infectious period → Non-infectious
o Susceptible → Infection → Incubation period → Symptomatic period → Non-
diseased
- There are four different cases:
o Index – the first case is identified
o Primary – the case that brings the infection into a population

, o Secondary – infected by a primary case
o Tertiary – infected by a secondary case


- For person-to-person transmission, you can calculate the attack rate of a particular infection:
people ill
attack rate=
people exposed
- Disease is the result of forces within a dynamic system consisting of:
o Agent of infection
o Host
o Environment
o
- Factors that influence disease transmission:
o Agent o Environment o Host
 Infectivity  Weather  Age
 Pathogenicity  Housing  Sex
 Virulence  Geography  Genotype
 Immunogenic  Occupational  Behaviour
ity setting  Nutritional
 Antigenic  Air quality status
stability  Food  Health status
 Survival

- Infectivity is the ability to infect
no . of people infected
¿ x 100
no . of people susceptible

- Pathogenicity is the ability to cause disease:
no . of people with disease
¿ x 100
no . of people infected

- Virulence is the ability to cause death:
no .of deaths
¿ x 100
no . of people with disease
- Chain of infection:




- Robert Koch (1843-1910)’s postulates is used to identify the microbial cause of specific
diseases. To do this he used a microbe that was present in every case of the disease. He
isolated the microbe from the disease host and grown in pure culture. The disease must be
reproduced when a pure culture is introduced into a non-disease susceptible host. The same
microbe must then be isolated from the diseased experimental host.
- Ecological factors in infections:

, o Altered environment (Air conditioning)
o Changes in food production & handling
o Climate changes
o Deforestation
o Ownership of (exotic) pets
o Air travel & exotic journeys/Global movements
o Increased use of immunosuppressives/antibiotics
- Endemic:
o Transmission occurs, but the number of cases remains constant and maintained in a
certain area or population without external inputs.
- Epidemic:
o The number of cases increases in the population in a short period
- Pandemic:
o When epidemics occur at several continents – global epidemic