Picking up the tab for erythropoietin.

model ofagent, host, and environment.28 Research that allows for interplay among these factors is likely to be most productive.29 We already know much about the environments in which multiple sclerosis is common and about relevant host factors (including those with a genetic basis). The agent(s) remains a mystery, although an infectious agent has long been suspected. More than 20 viruses have been linked with multiple sclerosis over the years, but in no case has the association proved causal.30 Recent reports implicating a retrovirus have not been confirmed.3'"33 Alter et al suggested that multiple sclerosis might be an age dependent host response to measles and other childhood infections.3435