@Zblurth They're under a lot of stress from just learning all of humanity was wiped out. Not the best state of mind to be in.
Most viruses only infect one species and it is rare for them to cross over to humans. Viruses adapted to aliens with completely different genetics would likely not infect humans at all. They mentioned that they can detect lifeforms, so with their level of tech they could probably detect harmful microbes.
They were one of several expedition teams sent to scout, not colonize, various planets. That's why they didn't bring people or sperm and eggs. Presumably, once the expedition teams returned they would decide which of the planets was best to colonize and send a colony mission to that one.
But most importantly, this is clearly not hard sci fi, the focus is not on technology or accuracy to real life. It's just a fantasy with a future setting. This is like getting mad at star wars for having ftl travel.