(Capcom's primary storyline)
Plaga A is the name given to a stage in the lifecycle of a Plaga (Spanish: "Las Plagas"), a parasitic species native to Spain. The Plaga develops into its "A" stage after hatching, later moving on to the "B" and "C" stages.
In this stage of their life, Plagas have soft bodies resembling flesh which pulsates, with yellow eyeballs and long tentacles. When they detect a threat they split their bodies in half and expose a giant long tentacle with a scythe like end to it, where they violently swing at enemies.
They are the weakest form of Plaga and can die with very few shots from a handgun.
Although not advised, Plaga A can be knifed. Slashes must be executed either on its center (host's head) or on the schyte-like tentacle when it attacks, which requires perfect timing.