There’s a particular trait Pokémon you can get in Pokémon Sword and Shield called Pokérus. It first sprang up during Pokémon Gold and Silver. What it does is provide the Pokémon with double EV growth rates, which makes them incredibly durable.
Traditionally, you have a slim chance of having a Pokémon obtain it after every Pokémon battle. Data miners previously recorded the chances of it happened somewhere near a 1 in 21,000 chance, which is a crazy attempt. Even Shiny Pokémon have a standard opportunity for showing up with a nearly 1 in 4,095 opportunities.
The best method to acquire it is to trade with another trainer who has a Pokémon with it if they are willing to do it, of course. When you have a Pokémon with Pokérus in your party, you want to keep them your roster and start to battle other Pokémon. The Pokémon adjacent to the infected Pokémon has a chance to contradict it following each fight.
You need to act quick, though. Pokérus does not stick around forever. You’ll know when another Pokémon has it by a small icon called “Pokérus” on their status summary, beneath their combat stats. It should have a little skull and crossbones icon.
Within a few days, the infected Pokémon gets cured, and they cannot obtain the infection ever again. But only while in your party.
The best solution to ensure you always have a Pokémon has Pokérus and then leaving them in your PC Box. They should always keep the disease while in your PC Box, and then pull them out when you’re ready to spread it to others.
It’s a useful disease to have, but you have to act quickly to capitalize on it.