Any vehicle that can cross interstellar space in a reasonable time also has enough energy to destroy a planet, simply by crashing into it.
So any alien race that can travel here could also drone strike earth and kill us all before we even saw it coming.
In fact, this makes it a prudent move to pre-emptively kill any civilization you notice, cause you can't ever know if they would do it to you, before it's too late. It's the principle of Mutually Assured Destruction, without the Mutual part.

As for diseases, we'd have to meet them in completely closed-off chambers.
And even then, it's possible that they emit strong x-rays to scan their surroundings, giving us radiation poisoning.
Or they use 200dB sonar, deafening us. Or the other way around, their bodies rapidly oxidize in an oxygen-rich environment.