Wesker's parents are Jeanne (from bayonetta) and captain america making him not only a male umbra witch but also part super soldier.in light of this, Jeanne spirited him away and entrusted Dr.Marcus with her son in exchange for the secret to eternal youth.Unfortunately, Spencer got to him, thus starting resident evil.
As for wesker needing a serum for his virus, its because the he got the t-virus and it messed him up.he was suppose to have died, got a sponser and rose from the ashes to kick ass without some last minute-plot-induced handicap..finally after falling into a volcano, wesker dies...only to find vergil in hell, accept him as his sponsor and return more badass then ever before with even more perfect hair (umbra witch) and magical shades that can see through bull****, so he never falls for anyone else's schemes again.
Then again...this could actually be an awesome fanfic...