Resident Evil 4
Set six years after the events of the Raccoon City outbreak, the Umbrella company has seemingly collapsed. Leon Kennedy is now working as an agent for the U.S. government and is sent to find the President's daughter, who has been kidnapped. His search takes him to Spain, where he finds a whole settlement under the influence of a mind-controlling parasite called Las Plagas. This is the work of cult leader Osmund Saddler, who Leon manages to defeat with Ada Wong - an agent seemingly working for Albert Wesker.
Links in the chain
At the end of Resident Evil: Gaiden, Leon is seen bleeding green blood, suggesting he has been infected by a strain of the T-Virus. However when we see him RE 4, he is alive and well.
The T-Veronica virus has similar qualities to the Las Plagas virus in Resident Evil 4, as the creatures infected behave in a hive-like manner obeying the orders of a "queen" or leader without thought.
Conclusion?
The mutagenic Progenitor Virus - the mother virus of the T-virus - was devised by James Marcus, Ozwell Spencer and Edward Ashford. So there.