Introduction to the Infectious Diseases

Question 1

If we have an immune system, how do we become infected?

Answer 1

The innate and adaptive immune response is not 100% protective
Microbes possess numerous characteristics to evade/overcome defences
The evolution of microbes is much quicker than host evolution (average generation time for bacteria (1 hour) vs 20 host (20 years)
Gene exchange (on plasmids/transposons) e.g., antibiotic genes, virulence genes

Question 2

Describe the host-pathogen relationship

Answer 2

The difference between a healthy state and a diseased state is shift in the response of the host to the microbe; the microbe’s ability to multiply, spread and cause disease vs. host’s ability to contain and eliminate. A delayed response can lead to disease and shedding

Question 3

What is adaptation and balanced pathogenicity?

Answer 3

For microbes (and host), it’s best to adopt in a way that it doesn’t harm the host. Over time, there is adaptation of both the microbe and the host. This leads to longer relationships without extensive damage. Example: Myxoma virus is spread by mosquitos and infects habits. In Australia in the 1950s, they introduced this virus to control the rabbit population. Initially 99% of the rabbits died, but more and more rabbits survived over time

Question 4

To cause an infection, pathogens must pass the innate and adaptive immune system. What are the different types of microbial infections?

Answer 4

Attachment of the microbe by itself and the gaining of entry into the host
Vectors (i.e., mosquitos) to bypass the skin and be delivered directly to the blood
Damage to the skin
Someone who is immunocompromised are much more susceptible to infection

Question 5

What are the steps of infection?

Answer 5

1. Attachment and possible entry into the body (how can we inhibit this?)
2. Local or general spread in the body
3. Multiplication
4. Evasion of host defences (capsules, hiding within cells)
5. Shedding form body (exit)
6. Damage to the host (not a requirement from the pathogens point of view, but it happens quite often, which is why we care)

Question 6

What are Koch’s postulates?

Answer 6

They are used in order to identify whether a certain organism causes a disease or not

1. The suspected germ must be present in every case of the disease (If it is missing, it is not the causative agent. It should also be missing from healthy individual)
2. The germ must be isolated and grown in pure culture
3. The cultured germ must cause the disease when it is inoculated into a healthy, susceptible experimental host (animal or plant)
4. The same germ must be reisolated from the diseased experimental host

Question 7

We cannot satisfy Koch’s postulates when trying to prove that HIV causes AIDS. How do we prove this?

Answer 7

We find all the people that are naturally infected with HIV and compare them to each other

Question 8

What are the challenges with Koch’s postulates?

Answer 8

Unculturable organisms (Treponema pallidum pallidum, cause syphilis) 
Host immunologic factors (e.g., autoimmune disease such as RA)
Involvement of more than one pathogen (there are cases where two pathogens need to come together to cause  disease; HDV cannot cause disease without HBV)
Infections only in humans (e.g., HIV)

Question 9

What are the molecular evolution of Koch’s postulates?

Answer 9

1. The virulence trait under study should be associated much more with pathogenic strains of the species than with nonpathogenic strains
2. Inactivation of the gene or genes associated with the suspected virulence trait should substantially decrease pathogenicity
3. Replacement of the mutated gene with the normal wild-type should fully restore pathogenicity
4. The gene should be expressed at some point during the infection and disease process
5. Antibodies or immune system cells directed against the gene products should protect the host

Question 10

What factors affect the severity of an infection? (the biologic response gradient)

Answer 10

Dose and route of administration
Age
Sex (e.g., women are much more likely to get a UTI than men)
Nutritional status
Genetic background

Question 11

What is the ‘iceberg’ concept of infectious disease?

Answer 11

Most infections don’t show symptoms (the immune system fights them off)
Sometimes we get an infection and the sypmtoms are a less severe form of the disease
Even fewer times do we get an infection showing the classical disease picture