I will get an email from friends now and again that offer me all the Viagra I can get or naked pictures of some celebrity and such.

What is this?
Its a virus.  (Spyware, Malware, Trojan etc)

I will then email that friend and tell them they have a virus and their email is hacked.

What should they do??

Scan their system, change their passwords.

I then tell them, as I am about to tell you..

Put your email address in all your contact lists.

Why??

Well, it will let you know, quickly, if your email is hacked when you get an email from yourself offering you guaranteed hair plugs or free porn and you don't remember sending it.
Then you can do the above stated actions before it gets out of control and send out the apologies to all your friends for the spam email.

Most people do not do anything for a while because they are unaware, and all their friends get mad when this stuff keeps flooding their email.

It will happen to you at some point as viruses are getting sneakier, hopefully not, but as the old saying goes....stuff happens.

Hope this helps!!