The Parasite is one of the most dangerous Swarm lifeforms because of the rapid maturation of additional Parasites inside an unwitting host. If a marine dies whilst infested, then four additional Parasites will be ejected from their corpse. The only way to remove infestation is to heal the infested marine (with the Medical Gun or Heal Beacons) until the yellow portion of their health bar is gone, or to activate Electric Armor. Healing Kits are very ineffective at curing infestation, but one kit can keep you alive for a few more seconds and give Medics more time to cure you.
In the Jacob's Rest campaign, Parasites first appear during the fifth mission, SynTek Residential, and onwards on Easy, Normal, and Hard difficulty levels. They spawn from Eggs or are placed in static locations on the map. When playing on Insane or Brutal difficulty, Parasites are encountered on all missions of Jacob's Rest except Deima Surface Bridge.
In the Tears for Tarnor campaigns third mission, Oasis Colony Spaceport, several drones have parasites riding them. If the drone is killed, the parasite will come off and attack the marines unless it's killed as well. They appear in several missions of other campaigns.
Contrasting from their in-game habitat, their biological habitat seems to require a moist area, be it in the air or any surface. This seems to be supported by the fact that the eggs they spawn out of contain an unsavory fluid.
- Parasites are viewed as a major threat to your marines. You can know one is nearby with certain audio cues from a marine that spots it, such as, "Look out! Parasite!".
- Try to kill Parasites before they make contact with a marine. The best way to do so is with the Flamer or the Grenade Launcher. Weapons with auto-aim are also effective due to the Parasite's small size. However, be careful while using the Flamer on a Parasite: its slow damage ramp-up can give the Parasite time enough to leap on a marine before the flames kill it. This is not the case with bullets.
- The chainsaw can instantly kill an airborne parasite if being used and aimed in the parasite's direction.
- After killing one Parasite, check the area for eggs and destroy them to prevent further Parasites from spawning.
- When only one marine infested, the Medical Gun is significantly more effective than the Heal Beacon at curing.
- If activated prior to coming into contact with a Parasite, Electric Armor will prevent any Parasites from infesting a marine for its duration. The damage from the armor kills Parasites instantly. If activated after a Parasite has latched unto a marine, it will take a few seconds for the armor to cure the infestation, which means marines with very low health may not survive without Medical intervention.
- If you have been caught by a Parasite and have a Blink Pack, use it. The Parasite will instantly die, and the poison, although it continues to drain your health, will eventually dissipate, so you will survive if you have enough health.
- If you wish to punch a Parasite, don't try to punch an airborne one; it is much safer to punch a Parasite from behind after it has landed because it requires time to turn around. Also it is safer to attempt punching parasite while electric armor is currently activated.
- The Parasite Puncher Achievement is awarded for killing a Parasite with a melee attack.
- The achievement Hat Trick unlocks a hat for Team Fortress 2 called the "Alien Swarm Parasite", which looks like a Parasite attached to the head of the player. The hat features jigglebone physics on the tail, which means that when the character moves, its tail will swing. As of July 2010, it is the only hat in Team Fortress 2 that features different positions on each of the characters' heads.
- When a marine dies from infestation, a huge explosion of blood appears and the marine falls to the floor with several Parasites flying out of the blood cloud. However, this expelling process leaves no marks or blood on the marine's body.
- The Parasite on the abandoned marine helmet in the Alien Swarm banner is much smaller than the in-game Parasites.
- A Parasite lit on fire may still jump and infect a marine, creating a burning Parasite graphic. Even if this lit Parasite disappears, the marine remains infested.
- The earliest parasite concept shows it was supposed to be snake like, movement being noted as "slithering" or "shuttling" across the ground, eventually springing up to a nearby Marine.