Yeh, was referring to how he managed to write a virus for an alien OS on alien hardware. Even got it to display gfx.
BUT I wouldn't think organisms are quite as specific- unless they operate with very different fluids/sugars/proteins which utterly lack important elements, they'd have to watch out. And no matter what they could run into trouble if they enter environments similar to their native stuff.
Virulence factors are pretty specific, actually. Regular-ass microbes cannot survive inside the body proper; pathogens primarily are able to work by subversion - hiding in cells, posing as something else... and even otherwise, they need to secrete stuff to adapt a body to their needs - that's why some bacteria are hemolytic, they need a fuckton of free iron and human body sequesters the shit out of iron.
For viruses, even more so - there's a bunch of lucky bastards who are immune to HIV altogether, because of a single mutation that changes a single protein the virus uses to enter the cell in the first place, rendering it unable to, well, do that.
And immune systems work on a whitelist, not a blacklist - it's either ours, or learned to be safe during development, or it's presumed hostile and ENGAGE KILL ERRYTHING.