An infected tile appears as a green ice block with lots of green bubbles.
In some levels there are virus spawners. When an ice block is over one spawner, it turns into an infected tile. An infected block, when hit, breaks only that ifnected block and will not destroy infected blocks also in the same direction as the block. If there is an ice block near an infected tile, the normal block turns into a infected tile. The virus can spread in every direction.
An infected tile (left) expanding the virus (right)