suddenly seeing a (fake?) virus warning popping up

I've never knowingly click a link in an email or anything but just this evening I'm getting a notification about a virus that pops up in the top right corner and looks just like when real notifications come, such as a software update. It just stays there for a couple seconds and goes away. It's trying to look like a real Apple notification, but I don't think it really is. I don't click it, but I am concerned. Can someone offer advice on this? Thank you.