It's interesting to speculate, though. If they were susceptible to our germs, or we to theirs, it would make an interesting problem. Hello, how are you, have some germs to fight. Not the best way to greet an alien species. I'm sure we would have plenty of infectious diseases to share with any alien we come in contact with. The only question is will we be able to treat them effectively? There's no knowing how an alien metabolism might respond to any antibiotics we might try to use on them. I think it's best we stay in our spacesuits and not share atmospheres with them. It would not be surprising to find out the first contact party had succumbed to novel bacteria.

If you don't care for reality, just wait a while; another will be along shortly. --A Rose