The hive is a communal parasitic alien lifeform spreading by taking over individual of other species through either burrowing inside the body of the host, usually through the mouth, and taking over its nervous system, or by implanting a microbial larva in their body.
Individuals are refered to as ganglions and look like a roughly insectoid shape, six to 12 inches long, with no exoskeleton, long articulated tendrils and a tail like appendage through which it can implant a larva in a host.
Right after implantation, hosts will often experience disorientation, voices, paranoia and extreme schizophrenic episodes. After approximately twenty-four hours, a control ganglion will have formed in the human brain stem and started to attack the host's central nervous system.. During this phase, the host intelligence is aware of its predicament, although most often incapable of action. During the year following the infection, the ganglion grows tendrils throughout the host’s central nervous system, rooting itself in it, dominating the host completely. At this point the host has become a low-level telepathic fully-functioning members of the Hive community and will tend to join other hosts to live in groups and clusters as this makes telephathic contact easier. After several years, it has become no more than a shell, a full member of the Hive’s collective consciousness, able to function on a global scale when enough members have joined the group mind.
When a host has been too severly damaged to function, ganglions with extended tendril networks have been known to be able to move out of their host and inside another individual to take over, usually making their way to the brain through the mouth. Full grown ganglion have been known to live inside an already hosted body, waiting for the controlling ganglion to find them a suitable host.
The Hive came to Earth in the 1940ies using the technology the stole from previously conquered hosts, Gray type aliens and asked for Earth's unconditional surrender. Their ship was shot down in Roswell , New Mexico. They have since then tried to establish a community on the planet in a more undergound manner.
Source: "Dark Skies" TV series