It might seem that just gradually increasing spawn rates and numbers of infected will create a nice smooth curve, but it turns out to be much trickier than that. Each type of infected has its own spawn schedule, so even though the individual spawn curves may be designed to be fairly easy at a particular point in time, sometimes they overlap in just such a way that the players find themselves in a really bad situation far earlier than we intend.

For example, a tank will be much harder to fight if a mob of common infected spawns at the same time - and nearly impossible if three hunters also show up right at that moment. A similar situation can happen due to variables outside our control - a team may not dispatch a tank or mob quickly enough, causing subsequent waves to start arriving and piling on to an already difficult battle. It's also hard to anticipate when teams will need to make an ammo run or heal up, both of which can be extremely difficult only a few minutes into the round. In order to help visualize these overlap points and the collapse of each spawn curve, we graphed out the wave behavior of infected over the course of a survival round.

The last variable we added was the "lull" - a short interval of time during which nothing new will spawn. Each lull gets slightly longer as the round progresses, but that's no guarantee the team will actually see the lull - it just prevents new infected from spawning. It doesn't help with infected that are already in the arena. Killing mobs and tanks as quickly as possible is the best way to ensure that a team can take advantage of a lull to heal, resupply ammo, or just move to a better location with more supplies. Given the extreme pace of Survival Mode, the number of zombies killed in a single round often outnumbers an entire campaign. Even optimizing towards using pistols to eliminate common hordes, ammunition usually becomes an issue at some point. As ammo piles provide a unique infinite resupply for players, they tend to be in relatively less defensible positions in the Survival arenas. This means making an all important ammo run is rarely a safe proposition and requires good teamwork and planning. Timing your resources correctly to be able to make a run when necessary can make or break a team. The perfect pipebomb or well placed molotov can mean the difference between a cakewalk and catastrophe.