Get uncomfortable.

The PG67A/W serum contains something resembling Uroboros.

The symptoms of the Uroboros virus are sweating, labored breathing, and delirium.

After taking the serum, Wesker exhibits sweating and heavy breathing:

The same symptoms can be observed in Wesker at Spencer’s estate.

Every episode of Wesker’s erratic behavior (when he attempts to kill Chris and delivers deranged speeches about evolution and the right to be a god) was preceded by an injection of the serum

The only encounter with a lucid Wesker occurs during the two-on-two fight. In this fight, Wesker behaves normally: he toys with Chris and does not mention his delusional ideas even once. At first glance, this seems like it breaks the pattern. But no: it fits perfectly - because this fight takes place AFTER the psychotic episode, in the window between two injections.

Wesker receives a dose at the exact moment when Chris makes his way through the ruins to the laboratory.

Chris then spends a long time moving through the laboratories - three full levels - during which Wesker experiences a psychotic episode. This is evidenced by an intercepted dialogue with Excella, in which she asks Wesker about his condition.