Infected Yahoo Mail Virus

A few days ago I opened up my Yahoo email account and began checking my mail. I opened an email that claimed to be from facebook, sure enough it was a virus that infected my computer. What pisses me off is how in the hell does this virus make it past yahoo mail and into my inbox! Don’t they have people checkin for this kind of stuff. It’s not difficult to figure out that a message is suspicious. If I noticed it, why didn’t they? Shouldn’t have opened it bottom line, but I wish Yahoo mail would do a better job protecting me from myself.