what factors are nesessary for a communicable illness?

Answer 1

it has to be able to spread person to person

Answer 2

1. The disease must be capable of surviving in the host organism. Not everything can be caught by a single species.

2. The disease causing agent must have a method to physically be transferred from one host to another. This might be indirect or direct.

An example of indirect transmission would be: I have the disease (e.g., Dengue Fever), I am bitten by a mosquito which catches the disease, the mosquito bites you, and give you the disease. The disease in this case is not directly person-to-person transmissible.

An example of direct transmission is: I have the disease (e.g., Influenza); the virus is present in mucous secretions in the lungs, I cough; the virus goes out in the moisture of my breath; you inhale the virus; you catch the disease.

3. In the case of host-vector transmission, the disease must be capable of surving in the vector long enough to be transferred. If the disease kills the vector fast enough, there won't be the opportunity to transfer to the next host.

4. The host's immune system must be susceptible to the illness. Some people have a developed immunity (have been exposed before); some people have a natural immunity (don't know why, just won't catch the disease).

5. While this doesn't effect the fact that something is communicable, a serious consideration is how fast the disease kills the host. If the host dies fast enough, there might be very little opportunity to spread the disease (for an example, see R. Preston's "The Hot Zone", but don't read it over lunch).