Zombies have been around since the beggining of minecraft. Knocking on our doors to let them in. And being a general pain in the butt. But as usual with my theories I will attempt to explain why zombies exist in minecraft.

I was reading a theory by Endure_Slayer on husks and strays when I thought "If these creatures are diseased then what about zombies?". Although when attacked the zombie does nothing but harm us and does not give us any effects. So the disease must not be spread by physical contact. It isn't spread by air or food. It isn't spread by any know way of a disease spreading. So how does is spread? Now this is going to go really deep so stay with me okay? Okay lets go. This comes a personal theory that I made a while ago. In no way related to minecraft.

I was watching 'The Walking Dead' and was trying to figure out how people become zombies when they die unless their brain is killed. Then it hit me like a snowball in winter. The virus was inside of EVERYBODY. It isn't active until the host has no imune system to fight back the virus or disease. It flows freely as it pleases through the body taking control of it. Another way of this virus spreading is if a person is bit by a walker, excuse me, zombie. It takes out the imune system letting the virus spread to the brain. In the movie 'Prometheus' I see this again where a parasyte weakens a persons imune system down. The person is killed though (with a flame thrower) and the virus spreads bring him back to life to terrorize the living. When we look in minecraft we see this happening to villagers all the time where the villager dies and is reanimated into a zombie.

I hope that was simple enough. And also that you didn't like I am crazy or anything. Thanks for reading!