Rare diseases do not always lead to restricted and shorter life spans. Sometimes they can actually help improve survival. 

Not everyone suffering from a rare disease faces a debilitated or shorter life. Consider Gilbert syndrome (GS), a hereditary condition that is usually mild and, ironically, a protective factor against much more serious diseases.

The syndrome is named for Augustin Nicolas Gilbert, the first to describe the condition. GS causes hyperbilirubinemia. This means higher amounts of the yellow pigment bilirubin can be detected in blood, which generally corresponds with jaundice, a yellow discoloration of the skin and the sclera. Additionally, the stool turns pale and the urine dark.

Most people have never heard of Gilbert syndrome, but they are familiar with jaundice or its technical term, icterus.

At the end of their life cycle red blood cells release hemoglobin, which gets degraded to bilirubin, a hydrophobic waste product. Toxic in high concentrations, the compound is transported to the liver for metabolic disposal. If all the bilirubin cannot be conjugated to glucuronic acid, some of it remains water-insoluble and cannot be excreted via urine and feces. This unconjugated form instead accumulates in the bloodstream.   

People with Gilbert syndrome inherit a mutation in one of their liver enzymes responsible for producing conjugated bilirubin. If less of this enzyme is available, the liver cannot always keep up clearing bilirubin from the body. Official statistics report that up to 7% of the population develop this condition, with males being more affected than females. When it is finally discovered, most patients carrying this mutation require no treatment.

But there is more reason for feeling relieved. Surprisingly, researchers found evidence that mildly elevated bilirubin levels reduce the risk of developing cancer and cardiovascular diseases. Both are ranked major public health issues worldwide. If confirmed by more epidemiological data, then a metabolic dysfunction could be considered a protective factor in prevention strategies. In this hypothetical scenario people with Gilbert syndrome would have an inborn advantage.