The thing about virus as a plural ( in No. 4) is that the correct Latin plural is indeed virus (with a long u), but this sort of parade of learning doesn't impress some readers (e.g. me) and doesn't communicate with many others (who'd assume that the noun was singular, which would under-value the software). Most writers about viruses use that as the plural.
Most technical writers would avoid the use of the passive here (1 and 2). It doesn't bother me, but the thing about the active voice is that it says what happened and when, and what did it - so it could be argued that using the passive paved the way for the 'this incoming message' problem. If you'd used the active voice, you'd have thought about when the check was done.
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