system
Given the following post, respond as if you're commenting on Reddit:
human
Why do Virus's evolve to be lethal?

I was playing the online game pandemic the other day and it got me thinking about virus'.

Surely, from an evolutionary point of view, a lethal virus is a bad idea. It infects a host and then kills it off, effectively destroying itself in the process.

If a virus evolved to be completely non-lethal, but still highly infectious, then it's survivability would be dramatically increased. Because it could theoretically infect the entire planet without anyone noticing.

So basically, why do virus' evolve to kill off their hosts? It seems like a poor trait to have from a survivability point of view.
gpt
Most successful diseases are non-lethal, or lethal over such an extended period of time that it doesn't impact on the viruses ability to spread.

In some cases a high death rate might occur because it's the side effect of something that offers a survival advantage, constant coughing may damage the lungs, and thus be liable to kill the young/elderly/unfit but coughing is also an extremely good way to spread to other hosts. Hosts have immune systems and in most cases the virus only has so long to spread to a new host before the current hosts immune system manages to beat it down to the point where it can no longer spread.

In other cases a mutant strain may appear that increases those effects and throws the balance of host survival/spread effectiveness out. Sometimes it'll go so far that the host will die before it can spread, sometimes it'll be just effective enough to spread whilst killing its hosts, but if it decimates the population it's in then eventually it'll run out of new hosts.

TL;DR: They evolve to be lethal because evolution punishes new bad changes, it doesn't prevent them. There's a reason why you'll catch a common cold/flu once a year but don't get the bubonic plague all that often.