The disease is transmissible by touch and by airborne particles. Breathing, eating, drinking water, through breaks in the skin, through the nasal or aural passages, all these can lead to rapid infection. Initial symptoms are vomiting and diarrhea, followed by increasing difficulty breathing. The disease affects blood proteins, and denatures hemoglobin thus destroying the ability of the blood to carry oxygen. Internal hemorrhaging occurs as the blood breaks down.
The infectious agent is a prion, a distorted protein molecule that is not destroyed by normal anti-bacterial and anti-viral procedures. Because of its small size, it can also pass through some medical filters.
The agent takes up residence in blood vessel cells as well as in the blood, and thus while transfusions can temporarily delay the onset or worsening of symptoms, even a complete blood replacement cannot free the victim from the disease. What is required to cure the disease is a specific intelligent "antibody" which can seek out and break down every single prion molecule present in the body -- to do this requires technology around level 17 (TL-H). The infectious agent is sufficiently close to an existing blood component that isolation and elimination is almost impossible at lower technology without also destroying the victim's own blood. Treatments mixing tartgetted anti-prionic agents with total transfusions have so far always been fatal.
The origin of the agent is a surface protein in the pollen of a pine-like tree that grows around most hot springs on the planet. Among the initial settlers were several with an otherwise debilitating blood disorder, whose blood proteins are sufficiently distorted that the plague is not fatal. Extreme evolutionary pressures led to a minor human race, who while relatively weak and lethargic can function normally in the presence of the agent.
"Fortunately" the nature of jump is such that the crew of any ship which carried the disease would not survive to complete a jump from this system.