In Metal Gear Solid 4 how does the foxdie in solid snake kill big boss

Foxdie is a man made virus that targets victims with a certain genetic code. The code is programed before injected into a carrier and it kills people by stopping the heart, so it would be passed off as a heart attack. This it a great weapon for discreet assassinations. So the pentagon (or whoever was responsible for stopping the FOXHOUND revolt) injected it into Solid Snake and it was programed to kill FOXHOUND members. But Naomi ( Whom injected him with the virus) tweaked it so it would kill Snake. However, it was believed the Solid Snake was given Big Boss's superior genes, when he was actually given the recessive genes, so it killed Liquid Snake instead because he got the superior genes. So, when Big Boss met with Solid Snake in the end of MGS 4, foxdie killed him because his genetic code is the virus's target.