My yahoo email is infected. How do I get rid of the virus?

There's a spyware infection in your computer that is sending those emails to all your contacts without your knowledge. Download and install this antispyware: http://www.malwarebytes.org/products/malwarebytes_free Update it before doing a full scan. After the scan, change your email password as it may have been compromised. Oh, take advantage of the 14-day trial version of MBAM Pro for free. It blocks real-time threats from inside and outside your computer. After it expires it automatically downgrades to the free version which is just an on-demand scanner.

Firstly, scan your system with the antivirus and delete the email that is infected. Your antivirus should be able to detect the virus that is present in your system and get rid of it instantly. If not, then get a better antivirus that is effective and efficient.

Grab a copy of Malwarebytes (update and run a full system scan) after this grab a copy of a well known free Anti-Virus like Comodo, Avast or AVG and do the same.

Yes, you should change your password. Then scan your computer with antivirus software.