Disclaimer: I don't own any of these characters. They belong to J.K. Rowling. I'm not making any money from this in any way.
Slash of the male/male and female/female kind will eventually be appearing in this story. If you don't like slash then please leave. Don't read what you don't like.
It first started with Dumbledore. Nobody knew what was happening. Nobody could.
From Dumbledore it spread to the Weasley's first infection Ron, then through the rest of his family, leaving only George and Bill unaffected. From there it slowly infected the rest of the Order of Phoenix. Apart from George and Bill, the only people left unaffected within the Order, were myself, Snape and Remus.
The only reasoning that we could come up with, upon reflection was that we were immune due to having built up our immunity against the Infection, so our bodies and magic rejected it. While not being pleasant, it was by no means life threatening.
The Infection, as we've recently been told, was so Dark, it only affected the Light.
Nobody knows how the Infection started, or who it traces back to – and no one is game enough to try and touch such a Dark bacteria to find out.
Not even the Dark Lord. Who was our first candidate for letting such an Infection upon the British Wizarding population, but as it turns out – even he wouldn't consider something so horrific – not even on Muggle-born and Half-bloods.
It was no surprise to me when I found that it could only infect the Light, or those who hadn't extensively used the Darker Arts, since the Lightest people I knew were the ones first stricken down.
Those groups who weren't infected didn't surprise me much either. The majority of Slytherins and a small percentage of Ravenclaws, many Aurors, a few Ministry workers who had worked with or been in close contact with Dark objects and obviously the Death Eaters and their associates. Surprisingly there were a few medi-wizards and witches who were also unsusceptible to the Infection and these people were possibly the most useful in the after mass of the first Infection.
When we realised that only Darker people weren't being affected, we realised the only choice was to band together – all of the people who were surviving the Infection and escape Britain. This included all people and animals, magical and non-magical alike.
We left our home country no less than four weeks after the Infection first became known for what it was.