Inspired by the history of Europeans coming to the New World carrying bacteria to which the native inhabitants had little to no immunity, I thought: "What if aliens visiting Earth carried their own sort of infection or infestation, to which we humans had no immunity?" Sort of a War of the Worlds scenario in reverse... But I'm sure that sort of thing has already been done by someone, aliens carrying virulent disease(s) humans don't carry. So what if the infestation were of nanites--what if nanites become a standard part of healthcare for any advanced technological species? (Just as hand washing and sterilization of medical instruments become standard at a certain point of development--once bacteria are discovered and found to be potentially harmful.) So that nanites are literally crawling all over (and inside) the bodies of high-tech aliens (or perhaps time travelers from Earth's distant future). What if these nanites potentially posed a risk to the human race?