Picking up the tab for erythropoietin.

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 Another lesson from the history of epidemiology is that diseases may be preventable before their causes are properly understood. Thus the Royal Navy knew how to prevent scurvy more than a century before the discovery of vitamins, and John Snow's work led to steps to control cholera long before the cholera vibrio was identified.28 Some optimists hope that the vaccination programmes against measles will lead to a decline in multiple sclerosis. That remains to be seen, but the search for environmental factors should continue.