Quoted By:
Lumi infected tenma with a syringe while she was sleeping, Lumi new she would infect the other girls and she knew Pippa would meet up with Lisa and infect her and the idol girls. Lumi is immune to infection because she got vaxxed (the actual vaccine, not the public one), which is why she didn't get seriously sick like the others.
Tl:Dr this is Lumi sabotaging idol