Dialogue between Martian researchers

The plot of the movie and narration multiple times emphasized that aliens were very advanced and spent centuries researching humans and preparing for the invasion. So how could it happen that during all these centuries none of them noticed bacteria ? Let's imagine how it could've happened.

Martian research station, Timp-Timp and his boss:

- Hey, boss, look at these 2 humans. One of them sneezed and the other started sneezing too. Looks like smth important, I think we should research that further. I will add this to my daily report.
- Nah, never mind, probably just some weird mating ritual of the humans. I don't think we should add such garbage to our reports.

One hour later:
- Boss, the sneezing guy went to a doctor, doctor gave him a pill and the guy stopped sneezing. And these doctors in general seem to play important role in human society. Why ? Definitely we need to study this further. I have a feeling it's smth big, might even impact our invasion !
- I told you, don't pay attention to that. Probably those doctors are just some crazy fanatic cult.

One hour later:
- Hey, boss, that doctor just took blood sample of a guy and is looking at the sample under the microscope. Why would he do that ? Is he searching for smth in the drop of blood on a microscopic scale ? Definitely we need to investigate this and understand what's going on.
- *beep* Timp-Timp, one more word about this and I will replace you with Tomp-Tomp. Your colleagues are busy with really important serious research, while you're wasting your time looking at sneezing humans.
Take Tump-Tump as an example. Recently he discovered that thing humans call "pornhub". Now that's really smth important and needs serious scientific research, not your sneezing humans !
- You're right, boss. Gonna grab some sfunji leaves for dinner and will join the pornhub research too.