What do viruses do?

Solution 1:

From the wiki:

The Virus is a special kind of entity existing in all agar.io modes. They appear as cell-like entities which are surrounded in spikes. When a cell of 133 mass or greater consumes a virus, they will split into many pieces, making them easy targets for other cells, but gain 100 mass. Players' cells not large enough to consume a virus will pass through it harmlessly, making it a safe haven for small cells. The virus can also hide a small cell inside of it, so small cells can use viruses to hide from large cells or plot a sneak attack, though, when a player's cell is larger than another player's cell and very near to 133 mass, he will not hide behind it, but rather be seen in front of it. Once a player has 16 independent cells, running into a virus with a cell of over 132 mass will result in the virus being consumed without additional splitting, only gaining 100 mass.

A new virus can be created by a cell by ejecting 7 blobs at a maximum by using the W function. When the virus is given the seventh blob, it will eject another virus in the direction the player is moving. This mechanic is often used to attack larger cells who would otherwise be impossible to consume for the player. Keep in mind that when a cell is in range to give mass to a virus, it is also in range for the ejected virus. Ejecting viruses at other cells has considerable risk of counterattack. Viruses that are closer to ejection appear swollen and have more spikes.

Solution 2:

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.