The Pokérus, short for "Pokémon virus", is a x2 multiplier that affects a single user at a time. The Pokérus stays with a user for 15 minutes before it passes on to the next user, selected from the users who interacted with the previous user.
The host is the currently "infected" user: the user with the Pokérus. A link to the current "Pokérus host" is shown at the top left corner of every page, among the links to the list of online users, your own User Page, the Event Rank page, and so on. The current Pokérus host is also named on the main page of the Super-Lite version of PokéFarm.
On the host's User Page, you will see the Pokérus status icon by the username. If you are the one with the Pokérus, the status icon will appear on every page near the links at the top left corner of the page.
What it Does
The Pokérus affects a single user's entire party and fields at a time. It lasts for 15 minutes. During those 15 mintues, all Eggs and Pokémon belonging to the user receive twice as much EHP or LUP from interactions as they would under normal circumstances; in other words, it is a x2 EHP and LUP multiplier.
Additionally, interactions done by the host during those 15 minutes will earn the host twice as many Interaction Points and Credits as normal. Interactions given to the host also give x2 Interaction Points to the ones making the interactions.
The Pokérus is affected by other multipliers and bonus days. For example, an Ultimate Account grants a x1.4 EHP/LUP multiplier. A Meta-X 2 grants a x1.2 EHP/LUP multiplier. When affecting a person at a single time, multipliers are multiplied together to reach a total multiplier. In other words, a person with an Ultimate Account, an active Meta-X 2, and the Pokérus on a x2 bonus day would have a total EHP/LUP multiplier of 1.4 x 1.2 x 2 x 2), or x6.72.
As for Interaction Points, the x2 multiplier from the Pokérus appears to apply before extra points from an Interaction Points bonus day are added. For example, if someone gives one of the host's Pokémon a Berry that it loves on a normal day, the clicker will earn 16 Interaction Points. If the same happens on a +4 Interaction Points bonus day, the clicker will earn 20 Interaction Points.
How to Get It
Every 15 minutes (XX:00, XX:15, XX:30, XX:45 server time), the Pokérus is transferred to a different user. The user must be currently online (in other words, has been active during the last 15 minutes). The Pokérus can only be transferred to someone who has interacted with the host. The more interactions that a user has given to the host, the more likely the user is to "catch" the Pokérus. The chance of a single online user catching the Pokérus is equal to the number of interactions given to the host by that user divided by the total number of interactions given to the host by all online users.
Interactions given before the host was infected do count towards this calculation, as long as the user who gave the interactions is still online. However, having the most interactions with the host is not a guaranteed way to get the Pokérus. While clicking more does make getting infected more likely, the Pokérus could still transfer to someone who has made as little as 1 interaction with the host.
It is impossible to catch the Pokérus twice in a row. It is possible to catch the Pokérus multiple times per day, but not back-to-back. At least 1 other person must catch the Pokérus in between repeat occurences.
If no one has interacted with the host or if there is some kind of error in the system, the Pokérus infection will return to Kolink.