The plague was worse than the bubonic plague was for Europe since that only had a 75% death toll. At the start of 'The Underground', Prince Kiet come back to Vixinte and find it deserted save for seven survivors. As the capitol city, Vixinte likely had a population in the hundreds of thousands if not millions.
The plague was genetically engineered, but got away on them, and any natural immunity to the plague was at such a low occurrence (~0.2%) as to be virtually useless to the survival of the species (natural immunity was though... overrated, as they would just die of blood loss and infection, if they survived the attacks anyway.) Due to the previous regime's ruthless nature, it wouldn't be hard to believe that quarantine methods including 'shoot-on-sight' orders would have been employed, therefore it is safe to conclude that the plague was nearly impossible to quarantine.
The plague ravaged the population that was tightly packed in the cities, starting with the poor in the worst conditions. Those who had aerial means of escape, did. Those who could STEAL transport, did, often manning ships far under capacity - and many city refugees joined Rama. 
The Plague appeared first in heavily populated area with bad sanitation. The plague was found in the urine, blood and saliva of its victims, and unlike popular beliefs, it was not airborne. However, simply being coughed on and coming in contact with the fine saliva mist exhaled in coughing and sneezing would be enough to infect another. The virus would incubate for about 48 hours before symptoms would begin to show. First stages are unknown at the moment, but in the last stage the Fox turned feral, and lashed out in violence towards anyone they saw. (Full blown it culminates in madness that lasts several days of chasing, biting and killing.) Only Fox were affected by the Plague. The ferals would be described now as fast moving zombies by Humans. Once the chain began, it ran quickly through the city center. The final phase of the Plague was that the ferals lost eventually coordination to the point of immobilization, and they died painfully from high fever and dehydration. As most of the Fox population was in Vixinte, the fox almost went extinct, and would have been uncoordinated, but because of Prince Kiet's absence during the plague, he could bring together his kingdom.
The weeks following the plague must have been a truly hellish nightmare with massive urban expanses turned into giant, stinking mausoleums. Imagine the fear in the hearts of country folk as word reaches them the only safe way possible - in the form of broadcasts by Foxes who know they are going to die. Some country Foxes don't even monitor broadcasts, so they'd need to hear from neighbors. Hopefully the plague will have died out by then or else they too will suffer and possibly spread it further. Imagine the reactions of the other races to the news and the speculation about whether any Foxes survived at all. Most of all, imagine how this would affect even the overly curious Blue Foxes opinions of biological weapons.
After the Plague, the Irincos led a full scale invasion into fox territory as the Fox were still trying to pull themselves together. Prince Kiet formed a quick alliance with the other races in order to drive them away from the Fox territory and from Cyantzium.