The Alien Disease is an infectious, highly-contagious disease caused by the Alien Virus, that affects humans and other closely-related hominids. The disease was engineered by the Alien Machines to be a swift and efficient method of eradicating humans on Earth.
The initial symptoms of the disease include fever, headache, coughing and general discomfort, among other things. Meanwhile, tumorous growth would frequently begin to develop within the tissue of the victim. An individual who has been infected for over a day will typically display deformities that run skin-deep. An infected individual after three days, however, will usually appear covered in blotches of a pale, moist fleshy webbing.
This swiftly develops into more apparent fleshy growths like mycelium that gradually grow and interconnect inside of the victim until they corrupt vital organs and expand around passages in the body. This kills weaker victims much more quickly than it does stronger victims. In fact, if allowed to grow continuously, these growths will come to fill most of the open cavities in the body, effectively reinforcing it.
In other cases, these tumors will form in an unusual way, and give-rise to odder transformations. For instance, the growths of victims who remain within a constant close proximity of each other may interconnect, causing the two to bind together and act as one.
The disease has more immediate psychological effects on the victim, such as an increased desire for social interaction with others of its kind, which ultimately develops into a primal instinct to infect and assimilate. Clearly, this symptom was intended to make infected humans more likely to serve as vectors of the disease.
The virus' "re-wiring" of the victim's brain will progressively cause their intelligence and applied thought processes to dwindle, replacing them with amplified primal senses and the basic need to infect others.