The "kyouryuu virus" (which I've been calling "dino virus" for the time being until Capcom announces an official English title for it) is a new type of status impairment/enhancement that can be transmitted from monsters to humans. When damaged by an infected monster, a gauge appears under the player's name. As time passes, the gauge slowly increases. Getting damaged by an infected monster also increases the gauge. Once the gauge is full, the virus goes full blown and the player loses the ability to regenerate health, which... kinda sucks.
However, there is also an added benefit that comes from infection. If the player can damage an infected creature sufficiently before the virus gauge is filled and goes full blown, the player can overcome the infection and is granted a bonus to their critical hit rate.
Capcom has not revealed specifically how creatures become infected but has announced that not all creatures start out infected and that some creatures can become infected during a quest... Someone call a vet.
Monster Hunter 4 is scheduled for release on the 3DS in March, 2013.