The game uses the tile ID to track the the viruses that exist. There are 4 viruses per tile, and the lower nibble of the tile ID describes which ones exist and are displayed. When the game calculates a generation this lower nibble of each of the surrounding tiles is used to calculate the new value for the next generation. The algorithm makes no special consideration for tiles that are adjacent to walls. The lower nibble for the tile ID of that wall tile is just used in the calculation. This is why some walls seem to behave a bit differently than others.

This patch is a graphics only hack that has stamped a representation of the equivalent virus state on top of the normal wall graphics. This in no way modifies the gameplay, but it allows you to actually see the state of the wall that is being used in the calculations.

Wall Viruses