emails (no subject)

It looks like something got into my yahoo email I noticed a few days ago there was quite a few MAILERDEMON notices in my inbox for items that could not be sent. I checked my sent folder and there was a lot of outgoing messages with no subject. I have seen those in my inbox from people I know but I know to never open them or click the link. This has happened now for the last three days with emails going out. Is there anyway to stop it? I am on a mac and don't have any virus protection. Should I get some?

Change your Yahoo password. Make sure your password is at least eight characters long and use numbers symbols capital and lowercase letters.

And totally off the subject, Earthworm Jim avatar = awesome.