Yes, they are. however the kharaa only needs to infect the biological partss to screw up the body and kill them, as im pretty sure their vital organs were biologacal. At least some of them.

Guy, do u know that some bacteria has the capacity of literaly eating some metals, like the bacteria that eats blood cells

You have to consider that humans are tough bastards disease-wise compared to aliens tho, there's so many diseases humans are immune from that aliens might never experienced and so weren't ready for.