Near the end of a video file I downloaded, it said the file was corrupted and asked if I wanted to continue. I said yes. The size of the file was over 100 MB so I know it wasn't just a spam ad. If it says it's corrupted, does that necessarily mean it has a virus? Or maybe the file didn't transfer smoothly?

Depends upon how much of the file has been corrupted. Not related to viruses afaik. The corruption means the download hash you downloaded the moment you chose to download, does not match up with the file that is downloading. So at some point LimeWire will attempt to redownload the parts that do not match.

Reasons for corruption could be one of many. Such as an over aggressive anti virus or other security program, a firewall, dropped packets along the transmission from source to your computer, perhaps the holder of the file changed some details of the file. Or their router is faulty. Another possibility is some kind of bug with the particular version of LimeWire.

As far as I know Limewire will attempt to redownload the missing/corrupt parts of the file up to six times before giving up.

In my experience, sometimes the file completes, sometimes it seems the file is somehow corrupted at the source .. or something is preventing the file transferring properly.