Why can some viruses/bacteria/germs only spread through certain methods?

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HIV, for example, can only spread through body fluids or the like, while the common cold can travel through the air; salmonella can be transmitted through contact with food. What differences cause each virus to only be able to be transmitted in their respective forms? Why can’t they all go airborne or the like? Why are they (seemingly?) limited to only those forms of transmission?

In: Biology

It depends on what parts of the body a pathogen infects.

HIV requires white blood cells to reproduce, which is why it only spreads through what we refer to as “blood fluids” which are things like blood (obviously), semen, and breast milk. The saliva of people who have HIV does not typically contain the virus, so if someone sneezes, they’re not going to fling the virus into the air like someone with a cold would.

The common cold typically infects the sinuses and the mouth and throat, so when we cough, bacteria will spread through the air.

Salmonella infects the digestive tract, but it’s also a bacteria that tends to infect chickens. Improperly cooked chicken, and vegetables that come into contact with contaminated water will pass the bacteria on to humans. And if a human gets it, and doesn’t wash their hands properly after using the toilet, they can spread the bacteria to any food they come into contact with.

It depends on how tough the germ is, how many individual germs are needed to start the infection, and what its virulence factors are. Virulence factors is the collective term to refer to the various methods germs use to attack us.

HIV and many other viruses are very fragile, and start to break down easily if it’s too dry. To travel through the air, germs have to have a tough enough outer layer to not dry out, because then they will break apart/decompose. To cause food poisoning, the germ has to be able to survive stomach acid.

The number of individual germs needed to overwhelm your immune system varies widely depending on the type of germ. [This wikipedia page has some comparisons of just how different these numbers can be.](https://en.wikipedia.org/wiki/Minimal_infective_dose#Comparisons)

Some virulence factors are only effective against certain kinds of human tissues. One bacteria can cause totally different diseases depending on which part of the body it infects, such as *Streptococcus pyogenes* causing strep throat if it’s in your throat or skin infections if it’s on your skin. Some germs only cause disease if they are in one part of the body and not another.