I was playing Pokémon Ultra Moon and I was fighting against Cyrus when I decided to switch my Pokémon to (Normal) Necrozma. I saw a status condition on it which was odd because I already cured its paralysis. After taking a closer look I realized that the shade of purple was too bright to be poison. Then I realized it said "Pokérus" on it. I noticed none of my other Pokémon had Pokérus and that I never put a Pokémon I traded in my team ever since I got Necrozma. The last trainer(s) I battled was 2 Team RR grunts. How did my Necrozma get Pokérus all of a sudden?