Three.

P1 was so different from anything else because everyone, across the country, no matter how long they had been sick, got better at exactly the same time. Exactly. It made no difference if you had been ill for months or days, suddenly and for no known reason, the virus just died out. In everyone. A few unfortunate people with existing conditions were left unable to recover and still died, but almost all of the originally infected population got better virtually overnight. And no-one noticed. No one in the health organisations thought anything remarkable about the fact the virus died out in the entire population at exactly the same time. Later figures estimated only around 10,000 people were infected with P1. It wasn't the kind of virus that spread rapidly and you could almost consider it controlled. It was as if it was an experiment of some kind. P2 showed us that's exactly what it was. A trial run to see if in fact the fail safes built in actually worked. And they did. In P1 at least. Of the original 10,000 people estimated to be infected with P1 and not just a variation of normal flu, around a third died, leaving approximately 6500 people in the UK who survived. A couple had gone overseas, but the majority of the cases were in Britain. The bulk of the P1 survivors were in the Midlands. Ground zero if you like. The initial infection site was never discovered, as so many cases of P1 were dismissed as seasonal flu and nothing more. It wasn't until P2 came along people suddenly took notice and realised, P1 was anything but normal.