Diseases are biologically-caused illnesses. They are rated according to how they modify ability checks, how long the modification lasts and whether the disease is fatal.
The modifier is a negative number and the duration is in 10-hour periods. If the duration is followed by an exclamation mark, the disease will kill anyone it has infected after that length of time unless the disease is treated at a hospital.
One particularly virulent example is the Blue Plague which killed seventeen million Frontier inhabitants the began in FY 17 and would not end until 10 years later. The Plague was so persistent and pervasive that quarantines were established throughout UPF space until a cure was found by the Medical Services Organization. Five systems remain under quarantine for the Plague to the present time.
For example, a -10/D10! disease modifies every ability check the character makes by -10 for 100 hours. The victim will die after 100 hours unless he is treated successfully at a hospital.
Curing a disease requires a dose of Antibody Plus; if the attempt fails, the antibody plus dose has been used up and the disease has not been cured.