Milnods
Technically, it’s known as mutated pseudocowpox, but that’s a mouthful. So was ‘milker’s nodule,’ but that at least could be trimmed down into a two-syllable word that sounded vaguely medical. Slang’s a funny thing. This version is contagious, from either skin contact or fluid exchange.
Anyway, milnods are what can happen when cowpox jumps species to a DNA-based bovine-equivalent, then jumps again into a DNA-based sapient alien species. Yeah, there’s a lot of those out there. Panspermia, Precursors, ancient alien species that used the same particular form of food yeast on all their planets; take your pick. The point is that people all over the Galaxy absolutely have to worry about catching each other’s germs.
Fortunately, milnods aren’t usually fatal: they manifest as mild skin lesions, and can scar if not properly treated, but the disease is generally treatable by interstellar-level medicine and usually goes away on its own within a week anyway. Still, it’s a common enough and irritating enough disease that most planetary systems have health protocols in place for dealing with outbreaks. Which means that it’s a great excuse to quarantine Terrans or their ships, should the need arise. Humans are typically asymptomatic carriers of the mutated versions, which makes it even easier to justify a quarantine. Should one be necessary, of course.
On the other hand: it’s real easy to convince most non-Terrans that a Terran does have milnods, which means that it’s probably not safe to touch their gear. That can sometimes be useful, too, especially since milnods aren’t really dangerous, just mildly disgusting. People can get much more upset over mildly disgusting.