Not sure about viruses. Yes, ads have been used as a vector, and there are zero days, but I think mainstream OSes and browsers have pretty good security if you keep up to date? I suspect phishing attacks are a bigger issue.

It can be quite difficult to tell the difference between spooky coincidences and actual targeted ads. My money is on spooky coincidences, but it's a problem that many people imagine otherwise, and there's no way to prove it one way or the other because modern technology is basically magic as far as most users are concerned.