"Ensign O'Halloran" undergoes a genetic reversion
Genetic reversion was a term referring to an individual who had been genetically altered involuntarily reverting to their original state. It was possible for this to happen to members of Species 8472 who were unable to keep control of their altered forms.
In 2375, Commander Chakotay and Valerie Archer, herself a member of Species 8472 in Human disguise, observed the member of Species 8472 posing as the Starfleet Ensign O'Halloran undergoing a genetic reversion, slipping away from Human form and displaying the appearance of Species 8472. Archer asked Chakotay if he had ever reverted and when he responded when he had not, noted that it was O'Halloran's third time in a week and that he would be pulled out of training if it happened again.
Later, Chakotay and Lieutenant commander Tuvok captured another member of Species 8472 posing as "Ensign David Gentry" and took him to the USS Voyager. When he refused to provide any information regarding his true nature, The Doctor tried to collect a genetic sample, but he released a cellular toxin and killed himself.
Later, The Doctor administered a cytokinetic injection and forcibly caused a genetic reversion, revealing his true nature. (VOY: "In the Flesh")