- "Yes, Rachel. You've been having an allergic reaction to the crocodile DNA."
"So what do I do? Is there some medicine I can take or something?"
"No medicine. At least none that humans could create. But there is a process. Something that happens naturally in these cases. At least it happens to Andalites. It's called hereth illint."
"That sounds poetic."
"A literal translation would be something like "burping DNA.""
"Now that's poetry."
- ―Ax, Rachel, Cassie and Marco
Because Andalites do not have mouths, they do not have words or phrases for "spitting out" or "vomiting up", hereth is the nearest equivalent.
When an individual acquires DNA to which they are adverse or allergic, they can lose some control of the morphing ability. This loss of control can result in spontaneous morphing and rapid morphing without transitioning through the individual's base form. The loss of control will continue until the offending DNA is expelled.
According to Ax, being allergic to DNA and performing a hereth illint are not common but not unheard of. However, the only known case of a DNA allergy among the Animorphs occured when Rachel acquired a crocodile.