Would alien viruses be harmless? Probably. Viruses can't reproduce on their own; they hijack normal cells to churn out copies of new viruses. So they're tailored to a specific type of cell. Sometimes viruses can jump species (from primates, livestock, and occasionally birds) but this is rare -- normally the viruses aren't able to hijack cells from a species they haven't adapted to. The possibility of alien viruses somehow being able to use human cells is almost nil.