What's to explain? The cancer cure was a genetically engineered virus. It worked. But once it got out of clinical trials, it mutated. The mutated strain caused the "rage"-like symptoms and aversion to UV light. This was really bad. And then it mutated again & went airborne. That was the end.
I thought the movie made the progression of the disease perfectly clear. That we only saw that through Neville's memories, as opposed to following the epidemic itself more closely, is kinda the point of the movie. It's not ABOUT the plague. It's about what happened AFTER the plague.
Also, what's with this attitude of "everything has to be explained?" I thought that went out with the 80's.
Anyway, really good movie, excellent performance by Smith. 8/10.