There's an outside chance the problem could be a virus, but the most likely culprit is your antivirus program itself or your spam filtering program. These apps work by situating themselves between your mail program and your e-mail server, then taking a look at every piece of mail you receive. But if one of these programs crashes or needs input from you, it will hold up the e-mail download and your mail program will think the connection has died. If this happens, just restart your antivirus program and spam filtering program and try downloading messages again.