Currently I haven’t named it, so this is just a placeholder, ZI. Zombie Infection.
It was unknown of its true origins although for the most part, the bacteria tends to hibernate in cold, frozen areas that were once tropical paradises. During short defrosts, sometimes these bacterias have the opportunity to move, but without way to transport themselves, they are almost useless. Which is true to many sicknesses.
As the planet gets warmer, defrosts become longer and more frequent, giving more opportunity for movement. These bacteria often hide in moving water sources (which freeze over, hindering their movement for however long) and they go down the stream to wherever their destination may lie. They also tend to inhabit carrion.
Almost all creatures can be a carrier to this infection. While wild and domestic animals tend to have lesser symptoms; they still have them. In humans, it’s more prominent. Symptoms are akin to rabies in animals. In humans, it’s a bit different (with some similarities)
In humans, ZI tends to stay pretty quiet. Kissing/physical exchange of bodily fluids is not advised, as ZI can be transferred in that way. ZI stays in the system for a very long time. It’s evolved to exist under extreme conditions for long periods of time, although it cannot survive fire. Humans might be aware that they have ZI, like ability to not be able to fall asleep as quickly or not at all. Lack of hunger might be observed, and injuries tend to stay for longer and get infected more easily.
Eventually, when the host does die, they stay dead for some time. Usually enough to be buried (unless they get cremated or something like that..goodbye, ZI.) but in rare cases ZI can activate within minutes after confirmed death. Depending on how long they’ve been dead, they have trouble remembering who they are. ZI can also bring back someone from the grasp of death, as long as they aren’t a skeleton. Reactions may vary, however.
Zombies can have varying degrees of intelligence, and even form societies together. They can even have ‘zombie pets.’
Zombies do not age. They can get injured, however. They heal after some time, but this takes a while.
Zombies are hard to kill. The almost (sure) way to kill a zombie is to immobilize it somehow, or burn the body (or both.)
There is no known cure for ZI. Preventive measures can be taken, like the usual (washing your hands, keeping clean.) It is also recommended to not eat raw meat or food in general. Wash your vegetables and fruits. Wear proper protection in areas with high ZI concentration. Stock up on food and have a radio at hand. If you have ZI, or suspect it, contact a professional.
Physical contact is questionable; is it safe or unsafe. Some have survived physical contact and remain human. Others do not. Vaccines are being researched.
ZI is nearly worldwide, even in deserts. In cold areas it goes dormant, making Antarctica a near safe place as well as the northern hemisphere.
Oh.
And mosquitoes are very common carriers, since they can take blood from a ZI infected host, and end up transferring it to their eggs or another person with (formerly) clean blood.
Attempts to weaponize the ZI virus are frowned upon. However, some groups have attempted/are already trying.