Gnezdo is not merely one creature but hundreds. In some subjects, the C-virus breaks down the victim's body into individual fragments that metamorphose into flying insects. These insects always remain clustered together after hatching from the Chrysalid and their swarm forms a vague humanoid shape, a shadow of the person they used to be, if you will. The Gnezdo swarm is controlled by a Queen, a larger insect with wasp-like features.
The Gnezdo swarm never strays far from the Queen and maintains a humanoid shape unless disturbed by an outside force. If disturbed, the swarm will scatter, but will soon recoalesce into their human shape. When threatened, Gnezdo attack by launching a slow-moving swarm at enemies. While slow, the insects are quite damaging as they will continuously sting victims.
Gnezdo can only be defeated by killing the Queen, which constantly remains enveloped by the swarm. Explosives are an excellent way to get the swarm to scatter and take longer to gather together again, leaving the Queen vulnerable. Shotguns are the most efficient weapons for dealing with the Queen quickly. When the Queen dies, the rest of the swarm dies with it.