It takes the form of a swarm of bugs that will chase and attach itself to the player character's head. While attached, it will decrease the player's Sanity and deal 1 damage every 10 seconds, starting 30 seconds after being attached[Verification Needed]. It will follow the player until it finds a non-fire light source to attach to, either at Dusk or Night (such as Thunderbirds, Town Houses, or Glowflies).
The swarm isn't a status effect, but is an actual entity that is flying around the player. When force attacking, the swarm can only be hit if there are no other targets around. Attacking them directly won't deal any damage, possibly because they are not a single entity but a swarm and therefore can dodge attacks quickly. However, they can be killed by other means:
- Spraying a cloud of Bug B'Gone,
- Freezing them with an Ice Staff, and then attacking them normally (this action does not decrease durability of the Staff but still drains sanity as usual),
- Leading them into the Gas Rainforest biome
If a Gnat Mound is destroyed, the swarm which comes from it will spawn a new mound after some time, similar to Moleworms.
- Overcharged WX-78 player should be cautious, as they shine brighter than the other light sources, and therefore the Gnat Swarm will never leave them on its own.
- When playing as Wendy, Abigail will instantly attack Gnat Swarms and won't stop doing so, unless the player goes into a cave in which Abigail is going to follow the player. After that, she follows the player as normal.
- A Gnat Swarm can be utilized as an easy way to fight Snaptooth Flytraps, as while a Gnat Swarm is harassing the player, the Snaptooths can only attack the gnats, leaving the player untouchable.