Some of the excessively optimistic tourists think they can avoid the plagues. They wash their hands constantly, carry hand sanitizer like a talisman against evil, and generally refuse to touch anything. Of course, eventually someone will cough in their vicinity, or they will have to reach for a railing to avoid falling.
Then, all those efforts are for nothing. Those people that are sick probably don't even realize it yet. Those that do realize probably don't care about infecting others. They handle every bit of the queues, touching props and jangling chains marking out the lines. They breathe the same air as you, standing much too close while shouting to people mere feet away. You will be exposed to the germs at some point, and eventually your immune system will break.