It turns out that Darumaka was actually the THIRD wild Pokemon I found with Pokerus! A Magnemite I captured in Diamond last year had Pokerus when I captured it, but I already had Pokemon with Pokerus in my party, so I thought maybe it got it from them. But a recent post in the Official Pokemon Topic proved that theory wrong:

If you have a Pokemon in your party with Pokerus when you capture a Pokemon, can the captured Pokemon get the Pokerus immediately upon capture?

 

No. You need to have a pokerus infected 'mon in the lead, and battle some wild pokemon. After a few rounds, it will infect the pokemon next to it... if you switch the second infected to the 6th slot, it infects the 5th, and the 3rd slot gets infected. (Speeds it up).