They don't. Diseases that kill people come from other animals. Cholera, for example, I believe is made for cows. It gives them the shits but nothing else. Unfortunately, a little case of the shits for a cow is fatal for humans

Viruses don't have brains, they don't "want" to do anything. Because they don't have brains, they don't "know" what is happening to the host - they can't react to what their host is doing.

Lots of times, say the flu, viruses kill because the hosts immunity system goes into overdrive. It can be the immunity system that is the actual cause of death.

Viruses don't "want" to do anything, as they lack the capacity for any type of cognitive awareness. Not all viral infections are fatal. When someone dies as a result from viral or bacterial infection, it's not necessarily the germ that's killing them, but the symptoms caused by said infection. If an infection causes inflammation in your lungs and you cannot breathe as a result, you die from asphyxiation. There's no "will" or "desire" from the virus, it's just how your body responds to a given infection.

NOTE: I am not a doctor, virologist, epidemiologist, or any other type of expert in this field. This is simply my understanding based on reading literature published by experts in the field. I'm sure I got some nuances and/or terminology incorrect. You'll probably get better information reading peer reviewed research rather than asking random people on the internet.

Viruses don't "want" anything. You're ascribing motive where there is none. Viruses aren't even alive let alone sentient. From an evolutionary perspective, it doesn't matter if your host dies once you've reproduced and spread to new hosts, so there's no pressure for a virus to evolve to be less deadly so long as it doesn't kill the host so fast that it can't spread.

As others have noted, our tendency to use "want" or "designed" around evolution tends to lead us to more intention from the process than is factual.

For viruses the primary issue is R0 ("R naught"), which is the number of people who become infected for every infected person, so an R0 of 2 means that if I catch that virus I will infect (on average) two people. Anything over 1 means that the virus will spread to more people for each generation of spread.

A virus that is highly contagious will have a high R0 value. If a virus has a high R0 and also kills people after they spread the virus, that's irrelevant for the virus because it has already spread. The severity of the virus will often correlate with its contagiousness because the symptoms go along with the method of spread -- for example, respiratory viruses make people cough, which spreads the virus in the air, and a particularly virulent respiratory virus may make someone cough/spread a lot and also kill the host. If a particular virus evolves to be "worse," and that means it spreads to four people instead of 2 and also kills the host in 20% of cases, that virus will still spread faster than the old virus, even if the host is now dead.

Thank you!

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