On this page you can choose a strain (or opt for a random one). Then people can bite you or you can get infected automatically. If you don't sign up for a virus, then people can't bite you and you can't get infected. When you carry a virus to the full term, a skin will unlock in your wardrobe, which you can take on or off whenever you want. So the virus is always temporary.

Thank you so much, really helpful :)

You have to go to the Volunteer Infection Program (also linked on the news page) and choose a virus if you want to opt in. This thread lists which viruses are which, and shows previews of the completed skins. [edit] Lol really late

- Thank you also :) I can't believe how helpful people are on here, I'm used to Neopets but this is ++++++ :)

You're welcome c: And you're signed up for my virus so I bit you! ^_^

- Thank you! :)