I called this your thread because you are the most active user here and because I assumed you were the creator because I didn't check. I hope that you saying that me and Northpin the same user with different accounts is a joke, but I'm really unsure of that...
And you answered nothing and missed all the points, it seems you didn't even knew about the Mycoid virus a few posts ago...
A virus, spreading from ship to ship, doesn't need to directly hit its target. This is not a bullet, but a virus (with fungi-like properties or something, the details are fuzzy for my old mind ^^ ) spreading from one Thargoid vessel to another. You hit one, it goes home, infect friends, that spread the virus more, etc.
I'm not sure if we have the exact specs of that virus by the way, like how long until inert, spreading methods, etc . Maybe some here have better and fresher info, but I can't tell you how the virus managed to spread. I can tell you that it did spread, that it was "devastatingly effective", and that you could still see some supposed aftermath of this at the Thargoid sites.
Everything you said here is not what is said in lore. You say that this virus (or are you saying virus in general? I really hope not ^^ ) has "a very short period of killing it's host"... I guess you are talking about incubation? Or the time between first symptoms and final stage? Anyway, if you have those informations about the Mycoid virus with a source, please share
We don't know how much of the Thargoids population it killed, was it 1%? 50%? 99%? What we do know is that it killed a very large number of them, enough to stop the war.
And they didn't even answer that, it would have been very easy for them to reverse engineer the Mycoid to infect Humans and spread it all over the bubble day 1 of their return. They didn't.
If Thargoids had unleash a virus on Humanity forcing us out of the bubble and killing a huge part of the population in the meantime, how would have Humanity answered?