The vocal cord parasites are a set of ancient parasites weaponized by Skull Face and his private military XOF, the main antagonists of Metal Gear Solid V: The Phantom Pain.
The parasites nestle within a host's vocal cords and heavily reconstructs their bodies to make their residence more comfortable, causing drastic physical and mental changes and unintentionally giving them super-powers. These powers are very inconsistent and dependent on the strain, but almost all strains provide the host with superhuman agility, durability, strength, and speed. Unique strains, such as Quiet's, Code Talker's, and The End's strains, rearranges their biology to make them similar to that of a plant (photosynthetical). Others provide altered pigmentation that allows invisibility or armored skin, and others provide pyrokinesis and a crippling thirst for water.
While it may seem like these parasites are a gift than anything else, Skull Face manages to weaponize them into extremely fast-spreading zombie-viruses, turning Mother Base into a living hell by having several outbreaks triggered, killing dozens. This is because certain strains merely want to spread, and do this by destroying their hosts cognitive functions and targeting any possible ways to breed, turning the host into demented zombie-like creatures. Code Talker, an expert on the parasites, states that humanity would be doomed if the virus were to spread to animals like birds. These strains were activated through speaking a certain language, and Skull Face planned to have the parasite spread worldwide so it would kill any English-speakers.