Introduction to Infectious Diseases

Question 1

What’s faster? Evolution of microbes or host?

Answer 1

microbes! Bacteria have a generation time of 1 hr vs 20 years for a host

Question 2

Adaptation and balances pathogenicity

Answer 2

Adaptation of both the microbe and the host over time should be done without extensive damage

• longer the relationship between the two - less damage that is done
  eg. myxoma virus - infects rabbits and is spread by mosquitos. Initially introduced into Australia in 1950 to control the rabbit population (99% died initially, but more rabbits survived over time). When a less lethal strain of virus replaced the original rabbits adapted

Question 3

What are the 4 types of microbial infections?

Answer 3

1. microbial attachment or penetration mechanism
2. biting arthropod
3. skin wound/ animal bite
4. infection due to antimicrobial defences being impaired

Question 4

What are the 6 steps of infection

Answer 4

1. attachment and possible entry into the body
2. local or general spread in the body
3. multiplication
4. evasion of host defences
5. shedding from the body (exit)
6. damage to the host - not a requirement by some damage is usually required for shedding

Question 5

What are some of the main challenges with Koch’s postulates today?

Answer 5

• sometimes have an unculturable organism
  (Treponema pallidum pallidum - the bacteria that causes syphillis)
• host immunologic factors (Rheumatoid arthritis) - rheumatoid arthritis results from an infection of streptococcus- caused by the host attacking its own cells. The disease is only a result of an immune response, and may not be reproduced in more than one person
• Involvement of more than one pathogen (hepatitis D virus has a high risk of developing liver cancer. Person has to be infected by hep B inorder to develop liver cancer from hep D)
• infections only in humans (HIV - ethically cannot infect other humans to determine whether or not the infection is the causative agent)

Question 6

What are Koch’s postulates?

Answer 6

1. The virulence trait under study should be associated much more with pathogenic strains of the species than with non-pathogenic strains (only a few genes that will make the organism virulent vs non-virulent)
2. Inactivation of the yen or genes associated with the suspected virulence trait should substantially decrease pathogenicity
3. replacement of the mutated gene with the normal wild type gene 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 7

What host factors determine the severity of the infection?

Answer 7

• dose and route of infection
• age
• sex
• nutritional status
• genetic background

Question 8

What were the original Koch’s postulates?

Answer 8

1. The suspected germ may be present in every case of the disease
2. the germ much 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
4. the same germ much be resulted from the diseased experimental host