There is many kind of viruses. Most of them does not damage Your files directly but many viruses can circuitously deprive You of use some files. Many viruses can even damage You hard disk drive and hardware.

 Moreover there is a group of viruses called Trojan Horses which allow other people to take control over You computer (to read files, send spam e-mails or delete data).

Another group of viruses can generate unnecessary and harmful traffic in Your computer network which can lead to lost data transferring through the network.

So we always advice to install the best antivirus security. 

You can red more about viruses and trojans in "Removing Viruses & Trojans" category.