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I tend to be fairly choosy in who I give my email addresses to.  I have a rubbish email address that I use for most websites that insist on an email address or for promotional offers that I know are going to be followed up by regular spam. I have another email address that I use just for financial sites and never use anywhere else. I also have a third email address for friends and family. Although I try to keep access to the latter limited, I have a few (dumb) friends who constantly respond to the frequent "alert emails" that warn of a virus and at the end ask you to forward the note to "EVERYONE YOU KNOW". I'm sure that these alerts are one of the means the spammers harvest addresses. If your friend has included you on the address list of one of these and forwards it on to you and the others, then there is the potential when one of the others likewise forwards the email on for your email address to be now in hundreds of emails of people you do not know.

Another trick is a machine that generates millions of email address (almost 99% invalid) and sends some email or other to all these people. Invalid email addresses will get a response from the server saying the address is invalid, so they can then exclude these. Of the others, they know they are probably valid but are not sure if they are actively used. Should anyone of these recipients respond to the email, perhaps simply to ask to be taken off the email list, then they know that those are both valid and active email addresses.