Adults have some level of immunity from viruses which help protect from becoming ill from their children or other menbers of staff. In a school, you have hundreds of them and it's a constant bombardment which easily gets through.
Also, from my experience, parents don't care if school staff get ill. I've seen pupils being sent in with projectile vomiting, temperatures of 104, chicken pox and meningitis, with the parents refusing to come and get them as they shouldn't be in school.
Adults just would stay off from work.
A ninja wouldn't let you know you'd been infected and you would only know for sure after the post-mortem.