History
When Janet Van Dyne was infected by a virus from the Quantum Realm, she infected her husband Hank Pym. He tried to take her out of the Quantum Realm which resulted in the spread of the virus around the world.
Scarlet Witch was infected by unknown means during the zombie invasion, Vision found a zombified Wanda and unable to kill her, he put her in isolation at Camp Lehigh where he would try to find a way to cure her even though her mutated state prevented her from being cured. Vision fed her to the survivors who received her signal of a cure for the infection only to be fed to the witch or Vision himself sought survivors among them T'Challa.
Vision's signal was received by other survivors who were his former companions and heard about the cure he was making for the infected. When Bucky went to check if there was a transport that would take them to Wakanda only to find the zombified Wanda and a bound T'Challa being used as food. Wanda unleashed a disaster on the base and killed some of the survivors, she was temporarily stopped by Vision when he collapsed the base on her. When she rose from the rubble, she attempted to bite Bruce but was repelled when his arm momentarily turned into the Hulk. Bruce forced the Hulk out to confront the zombies and fought the Scarlet Witch to give the others time to escape.[1]