Probably a country trying to create a bio-terrorism/warfare weapon.
But you'd think they'd realize the consequences; if one country is infected on a massive scale, people are going to try to escape, especially if they or a family member are infected. (The government will be trying to eliminate infected, hence the need for their escape.)
Thus the virus will spread across the world, eventually reaching the country that created it. Complete backfire.
Nature is somewhat likely, but not a parasite. Parasites need to keep the host alive so they can feed; zombies are dead flesh. Thus, even if a parasite fed on the dead flesh, it would eventually run out of food and die, because dead cells can't multiply.
A virus is more likely. Viruses and the like usually try to keep the host alive. If the host dies quickly, then the virus won't spread. So a non-human animal or plant would have to be a carrier of the z-virus. It may already exist, deep in the rainforest or in an unexplored section of the arctic, or deep in the sea.
I beleave its gonna be the goverment biological wepons countermeasures department that recieves a paracite and then thay try and kill it but make it stronger and i think that the goverment will cover it up untill we all know it
I think pharmaceutical companies have been poisoning us for years. With every flu outbreak and viral infection treated with antibiotics we further mutated these illnesses beyond what our bodies naturally cause. Eventually a super virus will occur with the ability to adapt and spread on such a mass level that our bodies even with medication will be unable to resist. Mutation on a cellular level to the point that even after death the body continues being a host, breading ground and carrier for the disease.
All it would take is the right virus, the optimal mix of environs and enough built up medications mixing up a genetic cocktail and there’s your apocalypse kindly administered with warm wishes from the FDA.