28 Weeks Later

During the original outbreak of the Rage Virus, Don (Robert Carlyle), his wife Alice (Catherine McCormack), and four other survivors hide in a barricaded cottage on the outskirts of London. They hear a terrified boy pounding at their door and let him in. A few minutes later, they discover that the infected have followed the boy to them. The infected attack and kill most of the survivors, while Don, Alice, and the boy are chased upstairs. Don pleads with Alice to abandon the boy, but she refuses. He abandons them as the infected break into their room by busting out of the window. Don desperately sprints to a nearby motorboat, narrowly escaping after a tussle with a recently turned survivor.
After the infected begin to die of starvation, NATO forces take control of Great Britain. Twenty-eight weeks after the outbreak, an American-led force, under the command of Brigadier General Stone (Idris Elba), brings in settlers. Among the new arrivals are Don and Alice's children, Tammy (Imogen Poots) and Andy, who were out of the country during the outbreak. They are admitted to District One, a safe zone on the Isle of Dogs guarded by the U.S. Army. Sergeant Doyle (Jeremy Renner), a U.S. Delta Force Sniper and his friend, Chief Flynn (Harold Perrineau), a helicopter pilot, are amongst the military presence in charge of guarding the district. Tammy and Andy are reunited with their father, who was found by the U.S. Army and has become the district's caretaker. In their new flat, Don fabricates a lie about their mother's death in order to conceal his cowardice.
That night, Andy dreams about forgetting his mother's face, so Tammy and Andy sneak out of the safe zone, and return to their former home, where they begin to collect family photographs and other mementos. To his shock, Andy finds Alice alive and uninfected in a semi-conscious, delirious state. The three are soon discovered by soldiers and taken back to District One. Alice is taken to a quarantine room, where she is tested and found to be an asymptomatic carrier of the rage virus. Don makes an unauthorised visit to Alice in her isolation cell, begging her to forgive him. She accepts his apology and tells him she loves him. However, when they kiss, Don is infected, savagely kills her, and goes on a rampage.
General Stone orders the building to be quarantined and District One to be put into Code Red lockdown. Civilians are herded into safe rooms, but despite the precautions, Don breaks into a room containing a large crowd and starts a domino effect of rapid infection. The mob - now with half its members infected - breaks out of the safe room and into the streets. Scarlet (Rose Byrne), a U.S. Army medical officer, rescues Tammy and Andy as the soldiers in District One are ordered to shoot indiscriminately. Doyle, unable to bring himself to comply with the order, abandons his post and escapes with Scarlet, Tammy, Andy, and several others through the Greenwich Foot Tunnel. General Stone orders District One to be firebombed, but large numbers of the infected, including Don, escape the bombardment. Scarlet informs Doyle that the children might hold the key to a cure and must be protected at all costs. Flynn arrives by helicopter to pick up Doyle, but he refuses to take anyone else, as they would be shot down for carrying people who are possibly infected.
Flynn contacts Doyle by radio and tells him to head to Wembley Stadium, but to leave the civilians. Doyle ignores his instructions and escorts Andy, Tammy and Scarlet to Wembley. They break into an abandoned Volvo V70 to escape nerve gas released to kill the infected, but are unable to drive away due to corpses jamming the wheels as soldiers intending to light the gas with flamethrowers draw near. Doyle exits the car and sacrifices himself by push starting the car and is subsequently roasted alive. Scarlet escapes an Apache gunship and drives Tammy and Andy into the London Underground, where the trio continues on foot. Don ambushes and kills Scarlet, then bites Andy. Tammy shoots Don before he can kill Andy, who remains symptom-free but an unknown carrier of the Rage virus. They continue to the stadium and are picked up by a reluctant Flynn, who flies them across the English Channel to France, as previously instructed by Doyle.
Twenty-eight days later, a French-accented voice requesting help is heard from the radio in Flynn's abandoned helicopter. A group of the infected are then seen running through a tunnel which, as they emerge into the open, is revealed to be the exit of the Paris MÃ©tro TrocadÃ©ro Station with a view of the nearby Eiffel Tower.

Question: Why do the infected begin to die out?
Answer: starvation