English Translation
In short, it is because the ‘plan.doc’ file I attached in the previous email was infected with a virus.
We never say ウイルスに「汚染された」 in the context of computer programs. It should be ウイルスに「感染した」.
ウイルスに「汚染された」even suggests that your computer is physically contaminated with virus, like a patient sneezed and he used your computer without washing his hands.