Why do you have an email address? If you never told anybody the address you are not receiving any mail, right? If you also never used the address as a sender, then you don't need that address either.
But if you ever used it for sending, written on your business card, on hand-outs, on scientific papers, on the subway wall or at any other private/public place then you will get Spam. Why? Because someone took the address and put it in his/her electronic address book. Even if they never used the address for past mailings but to make sure that they could for future mailings it is in their address book. And current - state-of-the-art - viruses/trojans grab these addresses from address books and mails in the sent folder and the inbox folders. Addresses found in these places are more valuable than addresses found on web pages because they are validated. With a very high probability these addresses are valid and reach humans. Who keeps invalid addresses in their address books?