The mineral selenium has been shown to be helpful in preventing a number of types of cancer (see Clark LC et al, JAMA 26 December, 1996, Volume 276, pages 1957-1963 and theLixian Study, Journal of the National Cancer Institute, 1993, Volume 85, pages 1483-1492). The dose of selenium used in these studies was 200 mcg (micrograms) a day. Doses of less than 400 mcg a day are considered safe, with toxicity only demonstrated with doses above 400 mcg. The exact mechanism of how selenium prevents cancer isn't clear, but there a number of other studies (largely epidemiologic, which refers to looking at populations and trying to establish correlations, which are not necessarily causally linked), which have shown patients dificient in selenium are more prone to a variety of cancers (e.g.Int J Epidemiol 1994 Dec;23(6):1127-32, Correlation of cervical cancer mortality with reproductive and dietary factors, and serum markers in China.).

Selenium has antiviral activity in varioius animal models of viral infections. I have used it extensively in people with recurrent cold sores, which are due to Herpes Simplex Virus I (HSV I). Using 200 mcg a day, it has worked increadibly well, with some people having no recurrences on it versus frequent recurrences before using it. It has been relatively ineffective in my experience with recurrent HSV II, which is often referred to as "genital herpes".

Selenium has also shown antiviral activity re: HIV.
Impact of a selenium chemoprevention clinical trial on hospital admissions of HIV-infected participants.
Burbano X. Miguez-Burbano MJ. McCollister K. Zhang G. Rodriguez A. Ruiz P. Lecusay R. Shor-Posner G.
Division of Disease Prevention, Department of Psychiatry and Behavioral Sciences, Miami, Florida 33136, USA.
HIV Clinical Trials. 3(6):483-91, 2002 Nov-Dec.
Abstract
PURPOSE: To evaluate the impact of selenium chemoprevention (200 microg/day) on hospitalizations in HIV-positive individuals. METHOD: Data were obtained from 186 HIV+ men and women participating in a randomized, double-blind, placebo-controlled selenium clinical trial (1998-2000). Supplements were dispensed monthly, and clinical evaluations were conducted every 6 months. Inpatient hospitalizations, hospitalization costs, and rates of hospitalization were determined 2 years before and during the trial. RESULTS: At enrollment, no significant differences in CD4 cell counts or viral burden were observed between the two study arms. Fewer placebo-treated participants were using antiretrovirals (p <.05). The total number of hospitalizations declined from 157 before the trial to 103 during the 2 year study. A marked decrease in total admission rates (RR = 0.38; p =.002) and percent of hospitalizations due to infection/100 patients for those receiving selenium was observed (p =.01). As a result, the cost for hospitalization decreased 58% in the selenium group, compared to a 30% decrease in the placebo group (p =.001). In the final analyses, selenium therapy continued to be a significant independent factor associated with lower risk of hospitalization (p =.001). CONCLUSION: Selenium supplementation appears to be a beneficial adjuvant treatment to decrease hospitalizations as well as the cost of caring for HIV-1-infected patients.

Most formulations of Selenium are derived from Yeast, and they smell and taste terrible. However, 200 mcg capsules are available. As usual with "dietary supplements", you should make sure any such product is USP certified.