(Covers information from Anderson's universe)
Las Plagas were T-Virus-based parasitic organisms developed in Umbrella Prime as part of development to create advanced bio-weapons just like other T-Virus-based bio-weapons such as Lickers following the discovery of a mutation that created Majini Undead. This particular parasite was developed to enhance its hosts into super-soldiers endowed with superhuman attributes and regeneration. If used on B.O.W.s such as Majini Undead, on the other hand, Las Plagas increases their intelligence to the point of capable of following orders. The latter function's discovery led to the creation of Las Plagas Undead, which then trained as soldiers in the event of an attack by external forces.
An improved version of Las Plagas parasite was used by Bad Rain in order to terminate Alice and her companion following the destruction of Umbrella Prime. The improved parasite drastically enhanced her physical attributes and granted her regenerative abilities that not only withstood gunfire without faltering, but could also expel the bullets from her body.