(Capcom's primary storyline)
The New York bioterrorist attack (tentative) was a viral outbreak which took place in across Manhattan. Perpetrated intentionally by A-GUA Industries, their A-Virus bioweapon infected a number of people in the Great Lakes and New York through the use of a branded drinking water. Hundreds, perhaps thousands, mutated into Zombies and fought with the NYPD. The outbreak was successfully contained and many of the Zombies were treated for the disease over the night.
A-GUA Industries developed the A-Virus as a means of causing massive destruction in the United States and Canada. Rather than a terrorist motive, A-GUA's motivation was from its nihilistic take on society inspired by its leader, Glenn Arias, following the death of his wife in a CIA operation.
The A-Virus was developed with the aid of Los Iluminados, who provided them with a Plaga for analysis. Unlike the t-Virus, which causes brain damage in humans during mutation and brings about murderous anger and cannibalistic hunger from excessive hormone production, the A-Virus instead mutates the host to behave aggressively towards only those uninfected, while retaining enough intelligence to speak. Three strains of the virus were created for the purpose of the attacks: the Type Ⅰ strain, which was spread by fluid transfer, specifically A-GUA's water bottles; the Type Ⅱ strain, spread in the air and engineered to 'trigger' Type Ⅰ to begin mutation, and finally Type Ⅲ which prevents mutation of Type Ⅰ hosts, and making them ignored by Zombies. If infected with Type Ⅲ soon enough, Zombies could also lose their cannibalistic tendencies without any brain damage.
Over several months, A-GUA spread Type Ⅰ across the Great Lakes and north-eastern United States in bottled water, so that in the eventual gas dispersal the virus could trigger mutation in millions. Per their plans, Type Ⅱ was only to be released once their mineral water brand was deemed to have sufficient customer appeal. In the event the number of Zombies proved too difficult for A-GUA to safely conduct activities, Type-III was produced in enough quantities for dispersal. After some weeks or months of A-GUA's water being sold, a series of cannibalistic homicides got the attention of USAMRIID, which were later deemed to be the result of random mutations in Type Ⅰ prematurely triggering Zombification in hosts without exposure to Type Ⅱ. The number of infected victims was more than 250.
After discovering USAMRIID was in the late stages of developing a vaccine and had, in fact, temporarily prevented Dr. Rebecca Chambers from mutating despite being an A-GUA customer, the plans were advanced by 24 hours. Specially modified A-GUA trucks were driven around Manhattan and parked on the roads. Simultaneously, they released clouds of Type Ⅱ, which mutated all customers enveloped in the fog. They quickly began attacking, killing, and eating others who had not drank the water. With the NYPD unable to hold back the Zombies and a sufficiently large policy reinforcement body being three hours away, the US government began to evacuate the general population. In spite of the US' usual restrictions on BSAA operations, the North American Branch was given clearance to assist.
As Type Ⅱ was being dispersed, the BSAA "Silver Dagger" team arrived in Manhattan. This elite team was aware of the Arias connection and Dr. Chambers' research, and had obtained inside information on the plan from a turncoat. The team succeeded in destroying one tanker, the high temperature flames destroying Type Ⅱ. Though the agents guarding the tanker were killed, this came at the cost of one BSAA member, killed by a B.O.W. dog. DSO agent Leon S. Kennedy was able to lure the dogs away from Cpt. Chris Redfield, though the pursuit caused a pile-up on the highway. Agent Kennedy and Cpt. Redfield made their way into A-GUA's headquarters in search of Type Ⅲ, fighting Zombies as well as A-GUA members. Arias himself was mortally wounded, but escaped death through mutation with his body merging with that of his right-hand man, Diego. He would finally be killed by a grenade, allowing the Silver Daggers to disperse Type Ⅲ across Manhattan.