The T-Plaga virus is a combination of the T-Virus and the Las Plagas parasite, seen in Resident Evil 4. After Wesker obtained the Las Plagas at the end of RE4, he started his experiments of creating the perfect B.O.W. In 2007, his experiments ended with the creation of the parasite.
The T-Plaga virus is very different from the orginal Las Plagas seen by Leon in Europe. The infected don't seem to feel any type of pain whatsoever. Also, they are more aggressive than the Las Plagas, whereas the Las Plagas would attack with caution is RE4, the infected are more probe just to charge into battle. They are known to eat human flesh, and are a bit slower than than their old forms. However, they are smarter, and are more agile.
Chris Redfield and Sheva Alomar (In an alternate Resident Evil 5 Universe) encountered the T-Plaga in Kijuju, after Wesker deployed them in an attempt to discover "The Jewel of Africa," also known as the Korbonos parasite, supposedly kept deep under Kijuju to pervent it from the reaching the surface like it did centuries ago.