So ordinary methods do work, it's just a simple matter of us ordinary humans not being strong enough to pose a threat. Fire is far from my preferred method of combat, but I suppose stylistic choices sometimes need to be set aside in favor of practicality. Endothermic reactions are much more difficult to weaponize without ethers, after all.

As for the virus, it's likely that it can be spread through any carrier. A typical computer virus doesn't care if its the original or a copy and can spread from any infected system, so it stands to reason that this would function in a similar way. Not to mention, it's simply best to err on the side of caution in cases like these.