Is Necator americanus approaching a mutualistic symbiotic relationship with humans?

The hookworm Necator americanus establishes infections of impressive longev ity in the immunologically hostile environment of its human host. In the pr ocess, it promotes pronounced T-helper 2 (Th2) cell activity, which in turn seemingly affords the host at least a degree of protection. Given the rela tively asymptomatic nature of infection, we argue here that Necator america nus might be approaching a mutualistic symbiotic relationship with humans. In our view, infection is controlled by the immune system while being suppo rted by a subtle immune-evasion strategy that is tolerated and possibly ben eficial to the host in certain immunological circumstances, such as in coun terbalancing potentially damaging Th1 responses.