This is a virus detection. Viruses are programs that self-replicate recursively, meaning that infected systems spread the virus to other systems, which then propagate the virus further. While many viruses contain a destructive payload, it's quite common for viruses to do nothing more than spread from one system to another.
Indication of Infection
Increase in size of infected files.
The date of activation for the damaging payload depends on the variant of the virus.
Variant - Date of Payload
.1003 - on April 26th
.1010 - on June 26th
.1019 - on 26th of any month
When the date condition is met, this virus will attempt to overwrite sectors on the hard drive and also attempt overwrite BIOS on flash-capable systems.
Methods of Infection
The only way to infect a computer with a file infecting virus is to execute an infected file on the computer. The infected file may come from a multitude of sources including: floppy diskettes, downloads through an online service, network, etc. Once the infected file is executed, the virus may activate.
Chernobyl, CIH v1.2, W32/CIH.Spacefiller, W95/CIH.1003a, W95/CIH.1003b, W95/CIH.1003c, W95/CIH.1003d, W95/CIH.1003dr, W95/CIH.1003e, W95/CIH.1003f, W95/CIH.1049, Win95.CIH, Win95/CIH.1003