Had a question about RE9

⚠️READ THIS FIRST⚠️: This is a lore related questions about RE9, so spoiler ahead!!!

How does Leon get infected with the T-Virus? Sorry, if I missed something in the game, but it doesn't make sense to me. Not to mention, was he infected in some previous game? (I haven't played or watched RE6, since the game was a bit of an eyesore)