I did not know about this and I have been playing Pokemon since its original western release.

There is a virus your pokemon can contract called Pokerus.

I was playing Crystal yesterday tapping 'A' in a pokecentre to get thought the dialogue, as you do, when I caught an odd line at the end of Joy stating they cant do more as they are just a pokecenter.

Elm then calls to talk about it but after looking in to it, the virus is actually beneficial and can spread to your other pokemon. It makes your pokemon stronger and dissappears after a little while but the improvement stays.

This phenomenon is reare then a shiny appearing with the odds being 1 in 21,845!

Did you know about this? If not, dont freak out like I did 😂

Images from google as I didnt think to get pictures till after.