The DDOS itself isn't a virus, but the DDOSers often use viruses to make it work.
It basically spams the server so much that the server wont work for the people that wanna play on it.
You need a lot of computers to do this, and one common and cheap way is to design a virus and spread it. The virus then makes your computer spam the target.
The target is selected by the person who orders and pays for the DDOS, which can be expensive.
So it all happens for a reason: someone want to bring our server down, on a friday night when you could expect a lot of players to come online under normal circumstances.
As our server is fairly high ranked on at least PMC, it is likely that it is a server of the same kind. They want to bring our server down, so more people will come to theirs.
Therefore, I suggest you all to stay off similar servers (especially if there's another teamPvP LOTR server). If they don't see the increase in playercount on their own server, they might get tired of spending money and stop DDOSing us.