If the sick one is in the rack or in close proximity (same room) with the others then they've been exposed. Bump their temps 85-95*F and keep them well hydrated.

The sick one needs to be isolated immediately if he isn't already, bump temps and humidity similarly. Always clean him after seeing to the others, then go shower. He also needs his own equipment - feeding tongs, spray bottle, etc.

QT should be at least 60 days for new arrivals, I do 90, I know some folks with boas who do six months.