I've recently written about extraterrestrial biohazards. The big issue: Do the ETs have 'normal' biochemistry or mirror-image biochemistry? If its the latter, then whatever bacteria they carry on or in their bodies would have a very difficult time infecting us. For example, we have mostly L-amino acids. If the germs have mostly D-aminos, they could not use ours as building blocks for replication.
A much larger issue for alien visitors with mirror image biochemistry: the photosynthetic microorganisms--like cyanobacteria--on the bottoms of their shoes. If these beasties find their way to the ocean, they'd be a problem in the intermediate term, because at the moment we wouldn't have anything that could eat them.
If the UFO aliens landed in Arizona or in Alabama, with their paranoid immigration laws, they'd probably get shot. In most other places, reason would probably prevail. If the aliens with mirror-image biochemistry take off their shoes, and put on some Earthly ones, there should not be a big biocontamination risk.