856 317 7272, 856 324 2025, both are legitimate Comcast Security dept phone numbers. The toll free is 888 565 4329. Of course Comcast could know when a user on it's own network has a virus. Do you think the virus just sits on your computer wrecking it for no reason? Nope, it sends spams, tries to infect other users around you or through your address book, etc. There are companies like ShadowServer who track DNS traffic through known botnet IP addresses

Legitimate phone number for Comcast abuse desk; see https://ws.arin.net/whois/?queryinput=!%20NET-68-56-0-0-1