Mixi was engineered to spread itself and kill as quickly and effectively as possible. The main vector for infection is through biting insects such as lice, fleas and mosquitoes, most typically through human lice. As well as this, the virus can lie dormant in dead flesh; disturbing corpses of those it killed can release the virus into the surroundings even after several decades.
Early symptoms of infection include red or watering eyes, loss of appetite, lethargy, and fever. In time, this progresses to cause lumps and swellings on the patient’s mucous membranes and skin. Most noticeably, the face becomes inflamed and scarred. As well as this, swelling around the eyes will force them shut, and similarly breathing becomes difficult as the throat and lungs become distended out of shape. Mixi infection commonly results in secondary infections taking hold, particularly respiratory conditions such as pneumonia. Usually, these secondary infections coupled with slow suffocation are what eventually kill the victim.
The comparatively high survival rate amongst British children after the infection hit the British Isles is an anomaly. It is understood by the Tested that those who survived were children who had received a trial vaccination for chickenpox, which granted a little protection against the sickness.
How the game rules deal with Mixi is covered here.