There is a special trait that Pokémon can have Pokémon Sword, and Shield called Pokérus. It first arose during Pokémon Gold and Silver. This allows Pokémon to double their growth rate in virtual vehicles, making them incredibly durable.
Traditionally, you have a low chance of having a Pokémon getting it after every Pokémon battle. Data Miners previously recorded the odds of this happening somewhere on a 1 in 000 chance, which is a mad attempt. Even Shiny Pokémon have a standard opportunity to show up with almost 1 in 4 opportunities.
The best method to acquire it is to trade with another trainer who has a Pokemon with them if they are willing to do so, of course. When you have a Pokémon with Pokérus in your party, you want to keep your roster and start fighting against other Pokémon. The Pokémon adjacent to the infected Pokémon has a chance to contradict it after each fight.
You have to act fast, however. Pokérus does not stay forever. You'll know that when another Pokémon has it by a little icon called "Pokérus" on its Status Summary, under its Battle Stats. It should have a little skull and crossbones icon.
A few days later, the infected Pokémon are cured, and they can never contract the infection again. But only during your party.
The best solution to make sure you always have a Pokémon in Pokérus and then leave them in your PC Box. They should always keep the disease in their PC Box and then remove it when you are ready to pass it on to others.
It's a useful disease, but you have to act quickly to take advantage of it.
