What do viruses do?

In context of Agario, viruses are a unique cell, not to be confused with a computer virus. They appear as cells with a dark, spiked circumference.

The Green Virus

The green virus is often used as hideout for smaller players, and an obstacle for larger players.

Smaller players will automatically hide underneath the virus. You can tell if a player is under a virus quite easily; When there is a player under the virus, the outside will exhibit "rips", as shown in the above picture.

If the player is approximately the same size as the virus, they will glide over the top. However, if the player is big enough to consume the virus, they will do so. While consuming a virus will raise the players mass, it will also split the player into smaller pieces, making them far more vulnerable.

If a player can consume a virus, but have already hit the maximum of 16 splits, they will not split. They will still consume the virus, and gain the appropriate mass.

Green viruses can be "fed", by ejecting mass into them. When they get big enough (the Wiki reports a maximum of 7 feeds), they will split into two separate viruses.

Wiki Inaccuracy

That said, this agar changes regularly. The information on the wiki is often outdated or inaccurate. At the time of writing this, there are no dark viruses in experimental mode. I personally checked this, accessing the various servers in spectator mode. Viruses react to comparative mass, not an exact number (you have to be bigger to consume, not exactly 133 mass). Also, I can confirm that you can feed green viruses in experimental mode.

A brown or dark virus is actually a mother cell. A mother cell spews 1 or 2 mass pellets for people to eat. When you're smaller than the "mother cells " you will die and gave the mother cell mass. When you eat the mother cell it works like viruses but give you 200 mass instead of 100. It works like viruses when you have the Maximus piece, 16, you can just eat them without exploding.