There are many species of extraterrestrials, all of which have different anatomy and physiology from humans, but pathogens can be dangerous for them.
Aliens live in unpolluted environments. Our planet is highly polluted! And suffers from a vast number of pathogens. Since extraterrestrials have not been exposed to human pathogens, they may become ill and infected. The anatomy and physiology of extraterrestrials are entirely different from those of humans. The life expectancy of extraterrestrials could reach thousands of years! Mainly thanks to perfect genetic adaptation to their environment, slow metabolism, energy conservation, and a clean, almost sterile living environment. Extraterrestrials, of course, know and are very familiar with the subject.
For military experts who would consider taking advantage of the fact I mentioned for biological warfare, I recommend abandoning the idea immediately! Aliens react sharply, quickly, and painfully to manifestations of violence against them. The result would be disastrous. Aliens have very sophisticated means of defense, using technologies unknown to humanity.