The feral wyvern virus warning shown at the end of a Monster Hunter 4 trailer is actually in the game and Tigrex spreads it. When you fight Tigrex an infection meter appears below your life bar and it fills up as you continue to fight Tigrex.
If it fills up your hunter catches the feral wyvern flu and that affects your natural recovery rate. You can reverse the effects of the wyvern flu to when you first caught the pathogen by repeatedly attacking the wyvern and defeating it.
Famitsu also has a few details about Goa Magara, a new black dragon-like monster. Goa Magara can sense things through its scales.
Monster Hunter 4 is slated for release in spring 2013 in Japan.