On Using No protection

I occasionally correspond with county councillors not of this ward. Yesterday I started receiving spam from a county cllr. I don't use anti virus software or malware lookout stuff.

I happened to open one of the spam emails nothing more.

Two hours later I notice I have started to send spam to people including customers from my address book exactly the same as the one I opened.

Ah ! you say you should have anti virus software. It would appear that such software has no use whatsoever for server side run operations.
( The Cloud )

So I changed my email password, that fixed it thankfully.
I also deleted all such emails.

So what did I and the county cllr have in common?

We both use Gmail
We both opened an email, nothing more.

I've informed said cllr using another  email address.