And, in fact, several intervention studies that provided sodium selenite in table salt or in tablet form showed that selenium supplementation could reduce the incidence of Keshan disease. The results were so impressive that the studies' control groups were abolished and given supplementation as well. However, selenium supplementation did not eliminate new cases of Keshan disease entirely, suggesting that the cause of Keshan disease is more complex than simple selenium deficiency. Because Keshan disease outbreaks have a seasonal pattern, some researchers suggest a virus may also be involved. Although selenium was associated with significant reductions in lung, colorectal and prostate cancers, it was also linked to increased risk of breast cancer.
Selenium has other viral connections as well.
Research done in the 1980s and 1990s showed that selenium deficiency might allow certain viruses to be more virulent. A Coxsackie virus, which causes heart lesions, was isolated from Keshan disease victims and injected into mice fed a low-selenium diet where it caused extensive damage. Supplementing the mice with selenium before inoculation reduced the severity of the lesions. So it is possible that to get Keshan disease, the victim must become infected with the virus while deficient in selenium. Whether concurrent vitamin E deficiency has an effect was not studied.