There is a particular trait where Pokémon can enter Pokémon Sword, and Shield called Pokérus. It first arose during Pokémon Gold and Silver. What this does is provide Pokémon with a double EV growth rate, which makes them incredibly resilient.
Traditionally, you have a slim chance of getting a Pokémon after every Pokémon battle. Data miners had previously recorded the chances of this happening somewhere near a 1 in 21.000 in 1 chance, which is a foolish attempt. Shiny Pokémon also have a standard opportunity to present themselves with nearly 4.095 in XNUMX opportunities.
The best method of acquiring it is to trade with another trainer who has a Pokémon with it if they are willing to do so, of course. When you have a Pokémon with Pokérus in your party, you want to keep them on your roster and start battling other Pokémon. The Pokémon adjacent to the infected Pokémon has a chance to contradict it after each fight.
You have to act fast, though. Pokérus does not stay forever. You'll know when another Pokémon has it with a small icon called "Pokérus" in their status summary, under their combat stats. It should have a small skull and crossbones icon.
Within a few days, the infected Pokémon are cured and can no longer become infected. But only during your party.
The best solution to make sure you always have a Pokémon has Pokérus and then leave them in your PC Box. They should always hold the disease while in your PC Box and then extract it when you are ready to spread it to others.
It is a useful disease to have, but you need to act quickly to capitalize on it.