There are many bad advertisements that can trigger a warning. So far there are few to no virus for routers so that's a dead end.
About all that I've seen is a DNS HIJACK but with thousands of router models there is no easy way to assure such a virus would propagate so my bet is bad ad on a web site.
I just got my laptop back yesterday from being scanned for viruses. The technician said it came up clean. I downloaded a Steam game. I can't remember if I was on Facebook or not, but I definitely didn't click anything! Yet today when I went to yahoo.com, I had a website open and my virus protection said it was a malicious site! I promptly exited using the x button. Could it be Yahoo's fault? Could my router be infected? If so, how do I clean it? A month ago I updated my router's firmware through an automatic process that was part of the router.
I have a desktop computer as well. I don't normally use a password to log on, so I said to the technician that there was no password. I realized later that there was a password for the administrator account. So, it would appear that he didn't clean my desktop computer with the administrator account. Could I have a virus from my desktop migrate to my laptop?