KV at first appeared to be effective with no side-effects, but over time, the virus began to mutate and cause rabies-like symptoms in its hosts.
The virus breaks out in December 2009. How the virus escaped from the laboratory is unclear.
Despite the military's best efforts to contain KV, it eventually mutated into a lethal new strain of measles with a 90% kill rate, and went airborne. KV spread out from Manhattan across the globe, killing 5.4 billion people. Of the 600 million who did not die from the virus, only 12 million were naturally immune to KV; the other 588 million devolved into Darkseekers.
There are two strains of the Krippin Virus: the airborne strain, which is spread through the air, and the contact strain, which is apparently spread through blood-saliva contact.
Once KV infects a human host, they will slowly succumb to the disease over the course of 48 hours. During that time, the host will suffer symptoms such as a fever, bleeding from the eyes and vomiting blood. By the time the 48 hour deadline is up, the psychological symptoms of KV will have surfaced: the host will develop primal behaviour and a vicious bloodlust. They will also see the uninfected as vicious monsters that must be destroyed, and will embrace the newfound power the Krippin Virus gives them. A human who fully succumbed to KV and survives is called a Darkseeker.
The contact strain of KV appears to overwhelm dogs much quicker than humans; it will only take a matter of hours before both the physiological and psychological symptoms of KV manifest in host dogs.
The Krippin Virus drastically changes its hosts' behaviour, causing extreme bloodlust and primal aggression. Infected hosts also suffer from albinism, total hair loss, and dilated pupils. Another major symptom is an extreme intolerance of ultraviolet which causes UV radiation such as sunlight to burn the infected's skin.
KV also causes it's host's body to constantly pump itself full of adrenaline, giving the hosts enhanced strength and agility, but also drastically speeding up the host's metabolism, raises their heart beat and blood flow, and cause drastic changes in the host's body temperature.
Of the 6.8 billion people worldwide at the time the Krippin Virus was released, only 12 million people (~0.002% of the original population) are naturally immune to the airborne and contact strains of KV. Dogs are immune to the airborne strain, but are highly susceptible to the contact strain. Rats are also susceptible to KV, whereas birds, deer, lions and presumably all other animals are apparently immune to the virus.
Compound 6 also gradually cures its hosts who are infected with KV; it cures at least the psychological symptoms and some of the physical symptoms in rats, and cures humans when they are kept at low temperatures.
- The Krippin Virus is very similar to (possibly inspired by) the Rage virus from 28 Days Later - Both are extremely contagious viruses that cause abnormal bleeding and extreme rage in primates (though KV can also infect canines), were created by scientists and re-engineered with another biological element (KV was meant to cure cancer and was mixed with Measles, the Rage Virus was meant to stop domestic violence and was mixed with Ebola), are extremely fast-acting (KV infects a host in just two days, while the Rage Virus can infect a host in nearly half a minute), caused massive devastation, and both the infected created by them have one significant weakness (Darkseekers are weak to UV Radiation, while the Infected are completely single-minded).