In general, I do not believe that we allow characters to die and then become Forsaken. If you create a character who
is a Forsaken, you must roleplay him as that from character creation. I will gladly defer to the wisdom of more knowledgeable individuals if I am remembering this incorrectly!
The original source of the Plague of Undeath was grain corrupted by Kel'thuzzad (at least in Lordaeron; I don't think the same method was used in Northrend for the initial tests among its locals), but it isn't the sole source. Before heading to Northrend, Arthas Menethil destroyed all of the original sources of the Plague. However, in the years that followed, remnants of the Cult of the Damned created cauldrons that spread the Plague throughout what is now known as the Plaguelands (Western and Eastern). They are capable of spewing the Plague directly into the air, increasing the likelihood of others becoming infected.
It is surmised that the Plague is actually some sort of fungus that can be transferred to others, which would explain how people can now acquire it. However, proximity may not result in becoming infected. There's a little bit of inconsistency in how it's portrayed in the lore -- what a surprise!
So, you have two options in creating the history of a Forsaken character:
- Your character can have been part of the original release of the Plague of Undeath in Lordaeron, which means that your character probably obtained the Plague from infected grain or from interacting with those who had. You then served for a period of time as a mindless undead servant until Sylvanas broke free of the Lich King's control in a moment of his weakness and then helped others do the same.
- Your character obtained the Plague of Undeath after these events. He probably either caught it while in the Plaguelands or interacting with someone who has spent some time there. At this point, your character was a mindless zombie until someone (usually a necromancer from the Undercity) broke the Lich King's control over him and then recruited him to join the Forsaken.
I hope that I have been able to shed some light on your questions, Kenert! If you need any other guidance in the creation of your future character's history, please don't hesitate to speak up.