There are two types of HLH. “Primary,” or “familial,” HLH is caused by an inherited problem of the immune system. “Secondary” HLH can occur when the immune system is disturbed (e.g., infections), but not necessarily because of an inherited condition. Both types of HLH are life-threatening. HLH can lead to liver failure, breathing problems, inflammation in the brain, and the inability to fight infection.
HLH happens more often in infants and children. But it can occur at any age, including in the teenage years and adulthood.
HLH is considered a rare disease. Statistics say it occurs in about one in 50,000 children. Because of that, many healthcare providers are unfamiliar with its symptoms, and it often goes undiagnosed for too long.