I assume that you are somewhere where the normal situation is that tap water is safe to drink.
The order not to do so can result from
- analyses (water samples) failing the microbiological or chemical testing
- breakdown/repair of a pipe, so that the water can get contaminated (the order for that is usually that you should leave the water running until the contamination is flushed out, and the water company will do some flushing also.
So there is a known reason for the order not to drink tap water.
Here (Germany) when the tap water is contaminated the news usually report the reason for not drinking tap water, i.e. they name the contaminant.
If the news do not tell that in your place, maybe you could phone the water company/city hall to find out what the reason is.
However, for chemical contamination "boil water for xxx minutes" usually woudn't help - they'd tell you to drink bottled water/water from the wagon they'll bring only.
Most cases we have here are caused by bacteria (tested are E. coli/coliform bacteria, Enterococci). The analyses AFAIK do not test for viruses.