Tainted beings are, as described by the Xenoblade Wikia, "abnormal variants of common creatures found only in certain parts of Noctilum (for the purposes of TARP, they can be found in other places as well). They attack and devour anything they encounter, save for other tainted creatures."
"Specimens include varieties of sphinx, caro, and scintimure that have been infected by a virus that induces madness. As this virus is unable to survive outside of Noctilum (ignored for TARP), the rest of Mira remains safely unaffected."
The Tainted are one of RipTide's many enemies. In his experience, he has found that they are incredibly difficult to destroy. Also, bodies of water and vegetation that packs of Tainted creatures stay near for extended periods of time tend to carry the virus.