How would Your Zombie Infection Start?

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My Zombie Infection would Start By Drinking Water Getting Diseased And Slowly The more People That Drink It , Turn Slowly.

The american cancer research society is funding work on a way to prevent brain tumors.To do their research, volunteers with said tumors are given Different types of medication their lab workers have come up with. One man, with an abnormally large tumor doesn't handle the medicine well, and is sent home due to violent illness. It starts out as horrible migraines and vomiting, and this goes for about a week.
At week two, he begins to notice his lips have a couple splits in them, wich he assumes are from the stomach acid. At this time, he stops leaving the house, not feeling well enough to even drive to the doctor.
Week three, assumedly from being layed up in bed, his legs and arms start becoming stiff. Headaches gone, brain numb, noticing blood in his vomit. He attempts to call a friend for a ride, cannot keep phone in his hands steady. Unable to make any recognizable words due to his throat being over worked from the sick. Shuffles around his house, moaning pitifully, and absentmindedly chewing his lips, despite the cuts.
Week four, three/five days in. No food, but vomiting is done. Lips are a mess, but cant stop chewing. Walking easier. Stiff, but possible. Finds himself watching the neighbors, because he forgot how to start his T.V. Begins leaving his back door open a bit. The breeze is nice at night.
13 days later. 3 a.m. Unable to sleep. Unaware of fact. Neighbors had party that night. It would be rude to not stop by. He gets up, and shuffles out the back door.