Though the researchers at Wildfire aren't sure what organism is causing the deaths, they use their limited knowledge of the microbe's behavior and symptomatology to test possible cures on samples in controlled environments.
One experiment they design comes after they notice that the microbe causes blood clots. Hypothesizing that these blood clots are causing the deaths, they expose the rats to the organism after trying to inoculate them against the clotting with varying amounts of an anticoagulant called Heparin. They keep all of the rats' other environmental factors constant to minimize the amount of statistical noise from other random variables in their findings.
The rats which receive a small dose of Heparin die instantly, and the rats which receive a large dose die after a slightly longer interval. This allows the scientists to conclude that though blood clotting is a slight contributing factor to the infection's fatality, it is not the primary cause of death.