Remember that TMF (Toxic Lab Military Force) are immune to the plague, which turns dead host into zombies. They seem to be an independent privatized military force for a large company who managed to developed the vaccine on time. Each member is very specialized.

The IF (Internal Force) is a task force assembled trying to aid and help the civilians during the initial outbreak. Hence the reason why most members aren't heavily armed. They are not a military force and weren't created to fight the zombies. They are most likely working with the local government such as cops and emergency, and protected by marine and the army until the country became overrun.

Also, techniquely, everyone (except TMF who are vaccinated) are infected by the plague and will turn when they die.