Throughout the comic and television series the cause of the sudden appearence of the Walkers has never been fully explained or revealed. However, there have been numerous clues left in both Fear The Walking Dead and The Walking Dead television shows that explain perhaps what exactly the walker plague is or might be. This is all of course pure speculation because Robert Kirkman has never specifically stated what exactly caused the outbreak other than whatever it is that does it is something that all humans have.
Which leads me to believe that whatever this thing is that it was not caused by some external force such as a bioweapon gone awry. The pathogen originates within the human body and that it is 100% communicable along with being fatal. My proof of this starts with The Walking Dead episode TS-19 in which Dr. Jenner says a few interesting things about what causes the reanimation process to begin with. In that episode Dr. Jenner describes the pathogen as acting like Meningitis which inflames the membranes around your brain and spinal cord. Meningitis's symptoms include a very high fever, headache, vomiting, nausea, etc. If left untreated it can lead to your death. Meningitis spreads through either viral or bacterial infection and given that it only affects you once your immune system has been weakened enough that the symptoms will become manifest in a person. Perhaps then the zombie virus is a new form of meningitis which has not be discovered by scientists. Further proof of this is when Dr. Jenner tells Rick that every human on the planet has this disease. This statement rules out the possibility of something such as a bioweapon from infecting the entire human race and makes its cause much related to some aspect of human physiology.
Another interesting clue is that throughout the show Fear The Walking Dead we see numerous individuals who are sick prior to becoming walkers such as Peter Dawson, the neighbor from the second episode who we see clearly sick as he wipes his nose demostrating that he has some unclear sickness and eventually we see him as a zombie before getting a shotgun blast to the head. Other individuals we see in the show who become sick prior to becoming a walker include Matt Sale, Alicia's boyfriend who is bitten by a walker off screen. One way meningitis can spread is through saliva, it is clear that it isn't the initial bite from a walker that kills you but it is what is in their decaying mouth gets into you that causes you to become sick and turn. Maybe it is the virus found in zombie saliva or bacteria from their mouth that causes the process of reanimation to take place. This would be further backed up by Dr. Jenner's failed attempt at identifying whatever the walker pathogen is. If it manifests like meningitis but doesn't show up as being meningitis then perhaps it has yet to be recognized by scientists which would make a cure impossible because if you cannot identify the pathogen how are you going to recognize it when looking at a sample of diseased tissue from a walker?
Now this is just my theory but it seems to fit given how little Robert Kirkman has said concerning the cause of the walkers. Everything he has said is that there is no zombie plague or virus which would rule out some bioweapon but leave open the possibility that this is a naturally occuring phenomenon that has yet to be identified by science but resembles something already known to scientists. What do you think?