Yahoo mail accounts are hacked regularly by spam mail rings so they can use your account and your contacts addresses to send spam. It is good practice to inform all your contacts your email account has been hacked (so they don't mistakenly open an attachment with malware) and contact yahoo support and inform them your account has been hacked (they can reset the password or close the account if you wish).
It is fairly easy for spammers to hack accounts with fairly weak passwords using a dictionary attack (a list of known words and passwords ran automatically) and even fairly secure passwords using Rainbow Tables, though it may take longer. For this reason it is good practice to use at least one email address only for private communication with people you know and keep other email addresses like yahoo and hotmail etc just for signing up to crappy websites and other junk. Using yahoo and hotmail for login on online shopping sites is not probably wise practice and also for communicating personal information as someone may break your password then simply observe your correspondence without altering your password. When picking an email address use random words and numbers, "[email protected]
" or something like that and don't post it on sites for the world too see as Bots scrape sites just looking for email addresses. Never use your name or Account name for email address as people will guess this, especially if it's at yahoo, hotmail or faceberk.
Even if you follow all the good security practices, your contacts won't, their computer will become infected and some worm will get your email address from their address book. Or one of your friends will bulk email your address to some idiot because they don't know how to use the Bcc: and Cc: fields when sending an email.
Use good security software and read up on personal computer security. There are some good applications to stop online tracking of your activities like VPN services and Tor, but you really need to study up on best security practices. Also ad blocking, web tracker blocking plugins for your browser, firewall that properly stealth's ports, good malware and antivirus.