An estimated 100 million people around the world are infected with Strongyloides stercoralis, a parasitic nematode, yet it’s likely that many don’t know it. The infection can persist for years, usually only causing mild symptoms. But if the immune system is compromised by the use of immunosuppressing drugs such as steroids or chemotherapeutics, for example, the parasite can reproduce uncontrollably, leading to a potentially life-threatening infection.