Dynamic infestation is a Kharaa bacterial growth which spreads from the Hive through a chain of Cysts created by the Alien Commander. Infestation grows and advances on surfaces around a cyst, and is the source of nutrition for Kharaa structures. Most Kharaa structures (except the Hive and uprooted Whip) will degenerate until death without infestation beneath them. Infestation also possesses several properties which enhance Kharaa defensive capabilities, most notably the ability to prevent Frontiersmen buildings from being placed. It is similar to the Marine's power network in that both are required for structures to operate, however the mechanics of each differ greatly.
Dynamic Infestation is spread through the use of Cysts and is a requirement of all alien structures placed by the Alien Commander. In addition infestation unlocks some useful abilities such as the Gorge's Belly Slide and allows support abilities such as Bone Wall or Nutrient Mist. While the Gorge's Hydra can be built anywhere, it will automatically build and heal itself if it is on infestation.
Dynamic Infestation can be seen as a network of interconnected nodes representing the territory control of the Kharra. "Pockets" or small outcroppings of structures cannot exist on their own. In other words, each structure must ultimately be connected to the hive though infestation. Because of this, territory expansion needs to be controlled and actively defended when dealing with control and placement of Harvesters as an alien commander.
Since Marines can place structures anywhere on the map with power, they have the advantage of being able to place structures quickly and in any location immediately. This means that a team of 2-3 Marines can effectively move from room-to-room, establish power, and build Extractors very quickly after the start of a round. This can give the Marines a large advantage very early in the game if not effectively countered. However, this also means that Marine Extractors are usually not as defended or watched as often as Harvesters once the round progresses. The Kharra's guerrilla warfare tactics are very well suited to take out these lone Extractors.
The Kharaa do not have this luxury, and must "branch out" from the initial hive towards Resource Nozzles. A common beginner mistake as an alien commander is to create very long "limbs" towards Resource Nozzles far away from the hive. If a Marine severs the connection between the hive and Harvester, the unconnected cysts and Harvester will start to die, and an inattentive commander may not take notice until it is too late. Thus, it is better to grow "organically" from the hive to the closest Nozzles first, and actively defend the territory expansion towards others.
Because the spread of Infestation restricts the placement of marine structures, it can be used to deny a part of the map from the marines. Infestation makes it impossible for the marines to place structures undetected as it must be removed first, warning all Kharaa. It is thus important to completely infest important rooms, leaving no clear floor where stealthy marines can place a [phase gate] or turrets.
Once Kharaa have resource superiority, it is often advantageous to extend the infestation across the entire map, including into the marine's final base(s). This both denies easy expansion from the Marines and also allows the use of Alien Commander support actions such as Nutrient Mist or Bone Wall. For the truly aggressive commander this then opens up the ability to construct or move whips in striking range of marine structures.