It would be completely unnecessary and far too risky for any alien species to invade us.
The biological transfer in both directions would likely kill both us and them.
An entire planet is far too large to be occupied in an effective manner. It would take hundreds of millions of alien troops for little net gain.
Any actual resources they might want are found in abundance in space.
Any intellectual resources could be pirated from our internet without landing.
Most behavioral changes could be forced through bombardment, containment and propaganda.
You’re more likely to catch the flu from a crocodile or Ebola from a bowl of petunias then you are to swap germs with an alien in any sort of harmful way.
Except that our very skin and breath are filled with bacteria, molds, etc. So we don’t even need to touch, just be in the same room to exchange or be subjected to theirs.
We have evolved to deal with the ones we have. Hell, we’re covered with bacteria that could well kill us, but our bodies deal with them in all but the worst cases. What another race could consider a harmless bacteria could wipe us all out.
It’s not about contact, the bacteria and viruses on Earth are all evolved to infect creatures with earth-like biochemistry. Aliens are going to be so totally different that there’s just no way that their microbes will be compatible with our cells. That’s why diseases like aids or Ebola can jump to us from other apes, and simpler viruses like influenza can even jump over from a bird or a pig, or how smallpox evolved from cowpox. Or something like rabies that can survive in both birds and mammals.
You never hear about a disease jumping from a cold blooded crocodile – or a fish or a plant – to humans. Their bodies are just so different from ours that the germs that live in us can’t live in them, and vice versa.
You’re misunderstanding what a disease is. Your skin is covered with bacteria that COULD cause you illnesses, but generally doesn’t, because you’re healthy and your body is used to dealing with it.
A totally, completely unknown bacteria being introduced isn’t going to be ‘incompatible’ with our biology. It will be incompatible with our immune system, which has never seen anything like it.
A microbe from another planet isn’t going to be a “bacteria”, though. It might not even be carbon-based. It likely wouldn’t recognize us meat-people as potential food.
Sure, it probably wouldn’t be a good idea to snort cultured Mars-laria, but that’s because our immune systems would probably recognize it as foreign and attack it. I mean, some people’s immune systems attack peanut proteins. Doesn’t make peanuts infectious.