20.Screening for HIV, Topic Page. U.S. Preventive Services Task Force. http://www.uspreventiveservicestaskforce.org/uspstf/uspshivi.htm
21.U.S. Preventive Services Task Force. Counseling to Prevent Tobacco Use and Tobacco-Related Diseases: Recommendation Statement. November 2003. http://www.uspreventiveservicestaskforce.org/3rduspstf/tobacccoun/tobcounrs.htm
22.Mayo-Wilson E, Imdad A, Herzer K, Yakoob MY, Bhutta ZA. Vitamin A supplements for preventing mortality, illness, and blindness in children aged under 5: systematic review and meta-analysis. BMJ. 2011 Aug 25;343:d5094. http://www.ncbi.nlm.nih.gov/pubmed/21868478
23.Arrieta AC, Zaleska M, Stutman HR, Marks MI. Vitamin A levels in children with measles in Long Beach, California. J Pediatr. 1992 Jul;121(1):75-8. http://www.ncbi.nlm.nih.gov/pubmed/1625097