AYes it is. Both the US edition, released in 2007, and the UK edition, released in 2008, are identical to the original DVD releases. Note that the film was also released in the UK in 2013 in a Limited Edition Steelbook version. This version has no additional special features.
AThe movie does not say. However, the graphic novel 28 Days Later: The Aftermath, which begins prior to 28 Days Later, as well as bridging the timespan between the events of 28 Days Later, and 28 Weeks Later, offers more details about the origins of the Rage virus. Two London scientists attempting to develop an inhibitor to control aggressive impulses in humans, determined that the best way to distribute this inhibitor was via a contagion. One scientist genetically modified the Ebola virus to carry the inhibitor, but the virus mutated and reversed the inhibitor's effects. As such, instead of working to suppress rage, it had the opposite effect, stimulating rage instead, and thus creating the Rage virus. After the scientists are forced to kill the first human test subject when he becomes out of control, they cover up the incident by burying his body in a field in the middle of the night. Subsequently, the two scientists have a furious argument and one quits. Later that night, he tips off the animal liberation group about the experiments, before shooting himself in the head. The other scientist becomes infected by a primate along with the members of the liberation group. An animated version of chapter 1 from 28 Days Later: The Aftermath, entitled '28 Days Later: The Aftermath (Chapter 1)', is available on both the 28 Days Later limited edition DVD and the 28 Weeks Later DVD. You can also see the animation here.
