Steven has never been sick, does that count? ^_^
Given he can bring people back from the dead, fix broken bones and regrow plucked flowers I think it's a fair bet he can cure sickness.  Which poses then the question what he's doing faffing around with Little Homeschool when he should be at the nearest hospital curing cancer! ?
bacteria and viruses for sure, as those are foreign invaders damaging the body
however, cancer is a bit more questionable as that one is caused by the body damaging ITSELF, but perhaps he still could as it is still damage just as much as a broken bone, faulty eyes, a cracked gem, or scrambled internal data are
That's my reasoning ?