system
Given the following post, respond as if you're commenting on Reddit:
human
How likely are we really to contract a virus or disease by drinking from the same cup?

Given two scenarios:

Your average US resident, sharing a drink from the same a cup of alcohol and also a cup of water (since I assume there would be less contamination in alcohol).

Your third world country resident, from lets say the Democratic Republic of Congo, sharing a cup of water and also a cup of liquor.

Personally what I think the whole "don't share cups" is ridiculously overblown, but I am curious.
gpt
It depends on the disease or virus and how it's transmitted.

If we're talking flu virus...don't share that cup! Herpes is also very virulent, but only at the site of infection. If someone has a visible cold sore, don't share that cup!

Some viruses don't spread very effectively through saliva, though they may be present. A famous example is HIV. HIV is very sensitive to the environment outside the body and can't survive for more than a few seconds in open air. If someone has HIV and isn't bleeding from the mouth, go ahead and share that cup :)