system
Given the following post, respond as if you're commenting on Reddit:
human
If somebody has a virus, but has very mild symptoms, is it because they have a weaker form of the virus, the person's specific immune system, or some other reason I'm not thinking of?

I know that back before vaccines people would say "Oh Jimmy has very mild X, go catch it from him so you'll get a mild case instead of waiting to get a bad case." Is that true at all?
gpt
Viruses reproduce by injecting their genetic material into a cell, which then does the work of making more viruses. There is no sexual shuffling of genes. So every virus produced by a cell is a clone. But errors do creep in and thus new strains are created. So there can be strains that are more or less virulent than the original. If the virus is really strong it will kill the host before it can spread, thus removing itself from circulation. If it isn’t contagious it will remove itself from circulation. So there is natural selection for contagiousness, and selective pressure for not being too deadly.  
  
The response of the person infected is the job of the immune system. So if yours is compromised, like you are already sick, or immunosuppressed, or old, you won’t respond well and might succumb.  
  
Thus there can be multiple strains with varying virulence in a population with variable response potential.