Background on Infectious Disease
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6 obligatory steps for microorganisms
- Attachment with or without entry into the body
- Logal or general spread within the body
- Evasion of host defenses
- Shedding from the body (exit)
- Cause damage in the host
What is the main reason that microbes are always a step ahead of the host's immune system?
Microorganisms are evolving at a much faster rate compared to most host organisms.
They multiply quickly due to very short generation time.
They also have horizontal gene transfer including antibiotic resistance genes.
4 main types of infection classified by characteristics of the microbe.
- Microbes with mechanisms for invading host body surfaces. - Viruses, certain bacteria.
- Microbes transferred via biting arthropods. - malaria, plague, typhus, yellow fever.
- Microbes transferred via skin wounds or animal bites. - Clostridia, rabies
- Microbes that are opportunistic and are only able to attack when the host's defenses are down.
4 main steps in Koch's postulates
- 1. The suspected pathogen should be present in all cases of the disease and absent from healthy animals.
- 2. The suspected organism must be grown in pure culture.
- 3. Cells from a pure culture are introduced in to a healthy animal.
- 4. The suspect organism must be reisolated and shown to be the same organism.
Exceptions to Koch's Postulates
- Some microbes cannot be cultured in the lab - Treponema pallidum, viruses
- For some microbes there are no susceptible nonhuman hosts.
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