How did people in the tv show called walking dead get infected?

I don't think they said howHow did people in the tv show called walking dead get infected?
We don't know the origin, but we do know it's a virus. An episode last season had a doctor explaining to the group how the virus works. It kills them, then the virus re-animates just some of the brain. Enough to turn the folks into little more than creatures who want to feed!How did people in the tv show called walking dead get infected?
I guess that they infected by a virus something like that, cuz if they will be bitten for some walker they change and made like a dead walkerHow did people in the tv show called walking dead get infected?
They didn't say how... so far