Pokerus is gone in the latest generation of Pokemon. Pokemon Scarlet & Violet just released today and players are discovering that there’s no indication of Pokerus anywhere in the game.
For those unaware of just what Pokerus is, it's a virus that was first introduced in the Gen 2 games. It's basically like herpes--any Pokemon adjacent to a Pokemon in your party infected with Pokerus will eventually catch it even though they're both still in Poke Balls. But unlike herpes, Pokerus is actually entirely beneficial. Pokerus will double the EV points the infected Pokemon gains from battles, and when combined with the Macho Brace item, can make training Pokemon exceptionally easy.
In past generations, actively infectious Pokemon would have a small icon that would indicate they were spreading the virus. You'd then place all the Pokemon you want to get infected beside that Pokemon and watch the icon spread over the course of a few hours (or even days in some cases).
However, that's no longer the case in Pokemon Scarlet & Violet. Not only is the icon for Pokerus infections gone, but there are also no assets for it in the game and it cannot be found on wild Pokemon (according to Twitter user Matt). He even checked with a known Pokerus-infected Pokemon and found that EV yield remained the same as non-Pokerus-infected Pokemon.
Scarlet & Violet still has the Macho Brace, so you can still get double EV gains, but the quadruple EV gains when combined with Pokerus are no longer possible in Gen 9.
Pokemon Scarlet & Violet is off to a bit of a rough start, technically speaking. Players are finding that the game can't quite maintain 30fps on the Switch, with some trainers saying they have to restart "every hour or two" to keep the game from dropping to 15pfs. This could be the surest sign yet that Nintendo had planned for a Switch Pro to be around before the release of Pokemon’s ninth generation.