You do know that gmail scans every word you type, enters it into a database and then sells advertising space to companies that target you specifically, don’t you?
If you send an email to a friend consoling them on a medical issue you will probably start to see Dr. adverts on the right side of your browser when in google for instance.
That’s why I always get ads at the top of gmail for spam and various ways to cook it to make it palatable.
I have never seen a single one of those ads thanks to Adblock, Flashblock and Noscript, though my gmail correspondence no doubt does have plenty of opportunity for ad harvesting.