Ok most of us have thought about this for years. I have watched countless zombie and other outbreak movies and have come up with a system I like but that might not work for every one.
Stage one <Insanity>: The outbreak of the z virus caused those infected to start acting strange, paranoid. They started to believe that everyone else was out to get them. At this point it is hard to tell they are infected at all. As it gets worse they start to lash out at those they see as a threat. Surprisingly they do not seem to attack anyone already infected and will even work with them to get people they do see as a threat. At this stage contact with bodily fluids has about a 90% chance of passing the infection. This includes but is not limited to any normal activity that would spread HIV, flu or colds. This stage lasts about two days with the noticeable behavior lasting about the last 6 hours. Often family members and people they come into contact with in the normal routine of their day are infected through casual contact. This explains how it spreads so fast.
Stage two<Feral>: At some point they will fall into a sort of short comma. When they awake they will have lost their humanity and are like the 28 days later infected. In this state they can not be talked to or reasoned with and will try to kill anyone they can reach. They will often start to bleed from the mouth and sometimes the ears and eyes as they near death from the virus. The parts of the brain that controls the organs starts to shut down. Pain drives them even more down the road of insanity and they start to ignore wounds that normally would slow them down if not stopping them cold. Three hundred pound men will run on broken ankles as long and the ankle will still support his weight. This stage can last anywhere from a week to a couple of months. What causes the wide time frame is unknown. In the end the victim will start to weaken and slow down. A combination of blood loss and the organs shutting down finally drop the infected and it dies.
Stage 3<Zeds>: Within minutes of death the body rises from the dead and starts moving again. All movement is slowed though sudden very short bursts of only a few steps are possible if they see someone close enough to grab and bite.
Stage 3 <bitten>: Anyone bitten by an infected or a zed bypasses the first stage completely and within hours falls into a comma. The over all health of the victim will effect the outcome. A reasonably health person will rise as a runner and will still be alive in all aspects. They will act the same as any other stage 2 that went through stage one. Progression from there is the same as well.
Unhealthy victims will die and then rise as a zed.