People in the city were affected by the infection, causing them to become violent (Search And Rescue). The water supply had become tainted (caused by Elena Derevko through using The Orchid). Also, the Mueller device broadcast a subaudible frequency. Anyone within the radius of the signal who drank the water was infected (Before The Flood).
News reports showed the riots and destruction of the city, but Russia denied any serious problem. When the APO team caught wind of the Mueller device above the city they realised it was the work of Elena Derevko enacting Milo Rambaldi's "endgame". Russia announced that they planned to detonate a bomb in the city to stop the infection spreading, not considering other costs of doing such a thing. Sydney, Vaughn, Jack, Nadia, and Irina Derevko (who was recovered from the hands of her sister to help stop the Mueller device) landed in Sovogda four hours before the supposed detonation (Search And Rescue).
When in the city, they met with Agent Brodien who was part of a DSR team that had already been sent into Sovogda. The rest of his team were dead and he told them more about the disease (Before The Flood).
Brodien told the team that tranquillizers would do no good against the infected. "They're like rabid dogs. They travel in packs," he said. Brodien said that it took them one night to wipe out his entire team. He estimated that there was hundreds, maybe thousands of infected. Everyone else was dead, and the infected had begun to turn on each other as there was no one left to kill. Nadia was troubled about just shooting the people, but Brodien assured her that they would be "putting them out of their misery". There was no way to reverse the symptoms.
The DSR team had deduced that the epicentre of the Mueller device was four miles northwest of APO's landing location, over the Oransky building. Jack understood that he meant the control centre of the device should be on the rooftop of the building. However, Brodien said that he had taken out the city's power grid, but the device continued to run. Irina explained that the device had been modified to run off an internal energy source. As the roads were blocked off, the team had to go through to subway to get to the building (Before The Flood).
As they reached the subway, Brodien was killed by one of the infected. Sydney and Nadia fired a round of bullets into him before he went down. Brodien in his last breath told Sydney to "end this." Sydney and Nadia then made it into the switching room of the subway. Inside, there were many dead bodies. Sydney and Nadia turned the lights on as Jack and Vaughn worked on starting up the subway train. Sydney arrived back on the train just as the lights came on, but was shocked to find Nadia was not there. Seconds later Nadia radioed in and said that the infected were heading towards her. They opened the back of the subway car and Vaughn began to shoot at thousands of infected that were chasing after Nadia. Just as Nadia caught up with the train, one of the infected leapt at her from the side and she was taken down and overcome by many of them (Before The Flood).
They made it to the epicentre of the device, and were contacted by Marshall and Weiss who said that Elena was trying to uplink a subaudible frequency to the Russian mil-sat network to distribute the infection world-wide. The second that the bombers were scheduled to destroy Sovogda, the frequency would be broadcast causing the effects of Sovogda to occur worldwide. This was, in essence, the "endgame". Weiss told them that they had around forty minutes to stop Elena. Marshall sent them building blueprints, but they realised that Elena would have every exit guarded. Jack told Sloane that he would help them get in, or he would shoot him. Sloane said it was possible to get to the roof, but stopping the device would be less easy. Irina explained that she could shut down the device by stopping the ionisation, and the only result would be the impact of the water (Before The Flood).
One of Elena's men found Nadia - who had managed to shoot all those infected that were attacking her - walking around the streets. He brought her to Elena, who showed Nadia a map of the world with pinpoints all over it, saying "the blood has begun, just a few will survive." Nadia refused to join her, saying that she would never kill Sydney as Rambaldi's prophecy had stated. Elena then infected Nadia with the infection with the aim of forcing her to kill Sydney (Before The Flood).
Elena noticed Sloane arrive back on the monitor. He was carrying a supposedly unconscious Vaughn, but as soon as they arrived in the base they began to shoot Elena's men. They were joined by Jack and Irina. They got to Elena and Irina knocked her to the floor with her gun. Sloane put Elena in a chair and Jack and Vaughn went to the computers to stop the mil-sat network from uploading the frequency. However, they could not retract the transmission (Before The Flood).
Meanwhile, Sydney arrived on the roof. Just as she got there, Nadia arrived. Sydney soon realised that Nadia had been given the infection, and she was guarding the device. Via comms, Jack and Irina urged Sydney to take Nadia out, but she said she could not do that to her sister. They fought for a short amount of time and Sydney managed to overpower Nadia, chained her away from the device's control box. Sydney managed to get into the main circuit board, but found a blue and a white wire. Irina did not know which to cut, and she asked Elena. When she refused to answer, she told Jack to start torturing her. Before he could do so, Elena told him to stop, and said to cut the white wire. Irina immediately shot Elena, and told Sydney to cut the blue, other wire. Just before she could do it, however, Nadia began to strangle her with the wire that she had escaped from. As Sydney was about to die, Sloane arrived on the roof and shot Nadia. Vaughn arrived and, with Sloane, took Nadia into the elevator as Sydney cut the blue wire (Before The Flood).
The Mueller device exploded, and water flooded Sovogda. The team made it into a safe location away from the water. Meanwhile, back at APO Marshall looked at satellite footage of the building and saw that it was no longer there, assuming the rest of the team was dead. They were alive, however, and in the streets Vaughn requested an evacuation from United States air support (Before The Flood).
The main difference in personality when someone becomes infected with the disease is greater violence. When the infection first broke out, people assumed it was merely riots, but it was discovered that it was more severe than this. The infected are not adverse to attack other humans to either kill or infect them (if one is bitten by an infected person one becomes infected themselves but they can, however, just kill their target). Furthermore, when there are no more uninfected humans to attack, the infected have been known to turn on each other.
Another difference between the infected and the original human is a more agile nature after exposure. This was evident when the infected were chasing Nadia down the subway tunnel. They also have a greater strength, demonstrated when one of the infected stabbed Brodien straight through his chest with a stick. People also appear to lose their ability to speak upon infection, and their mental faculties devolve to a simple bloodlust rather than thinking anything beyond that. The only infected person that acted at all differently was Nadia Santos, who squared off with Sydney Bristow for several moments before attacking her. Moreover, when she did attack her sister she did not immediately attempt to kill her or bite her; instead she spent time over the attempted murder.
Physically the infected differ from their human counterparts. Their eyes turn red and their pupils constrict greatly.