Yamphut is a sickness that results when disease-carrying organisms overwhelm an Andalite's Tria Gland, causing it to inflame and eventually burst, leading to death. It can also be passed to other species like humans, who exhibit flu-like symptoms. It seems that it can lay dormant in an Andalite for a long while, just as a certain type of cold virus that a human is immune to can be dormant in that human until it mutates into something the body is not immune to.