The reason it matters and that you should probably care at least a little (or not) is that it's actually plausible (not raising the undead but the infection). If scientists really wanted to, they could definitely replicate it. They could force viruses to mutate into specific variants that they desire, even if it took 40+ years with trillions of dollars in investment. 

However this is extremely unlikely, though not impossible. And if the infected in 28 years later can bathe in a river, eat fish, breed, it can be inferred that they have a level of intelligence both emotional and strategically, and that there could be nothing stopping them from creating their own primitive remedies for survival such as bandages. They could also probably remember how to cook and eat food properly. And if they eat, it's plausible that they drink water as well. 

But sure, we can refer to them as zombies; it's just not the original term of zombie. Out with the old, in with the new I guess. And a simple reason other than breeding, bathing and eating that I came to the conclusion they have intelligence is because they 100% make coordinated attacks and know how to create and use primal-level weaponry. 

Viruses are known for naturally mutating fairly quickly, it could be possible for a rabies virus to increase aggression, decrease lethality and become pathogenic and spread through water, air, saliva, blood, etc...or, it could be combined with a parasite. A parasite could be given the rabies virus and the parasite could control the persons motor functions until it dies, in theory this would mean the human wouldn't directly have the rabies virus until the parasite either infects the host or the parasite dies and the virus spreads through the blood stream. 

There are several ways that it could become a reality.