The incidence of primary HLH is approximately 1:50,000 live born children [8] . However, HLH incidence varies in clinical studies, most likely due to a difference in prevalence across ethnic groups and/or emerging awareness [9] . Although HLH often presents between birth and 18 months of age, onset in older age groups is possible and has been shown to be a feature of particular gene mutations [10] . Familial HLH has a median survival of less than 2 months after diagnosis if it remains untreated [9] . It is likely that many cases are misdiagnosed as severe fatal infection.