Health Resources Online

TeensHealth.org (http://www.teenshealth.org/) has information on diseases and conditions as well as other important information to help you live a healthy life!
WebMD's Health Page (http://www.webmd.com/) is a great site to look up diseases and find out about them. Also has health, fitness, and nutrition information.
MedlinePlus (http://medlineplus.gov) is a service from the U.S. National Library of Medicine and the National Institutes of Health, and has a Medical Encyclopedia (complete, easy to understand articles on just about *any* medical topic!) as well as prescription and non-prescription drug information, links to good sites about diseases and conditions, and a whole lot more!
Familydoctor.org (http://familydoctor.org/) has information on different diseases and conditions, tips for healthy living, a symptom checker, and medication information among other good health information. Site is sponsored by the American Academy of Family Physicians.
coolnurse.com (http://www.coolnurse.com) has information on mental health, sexual health, and general health and conditions. Great question and answer pages too!
Center for Disease Control (http://www.cdc.gov/) is the government site. Track epidemics, learn about advisories in other countries, and get facts about diseases from the government.
Yahoo Diseases (http://dir.yahoo.com/Health/Diseases_and_Conditions/) has links to information on all sorts of diseases and disorders: mental, physical, and genetic.
Internet Mental Health (http://www.mentalhealth.com/) has information about all sorts of mental disorders. Just click on the Contents tab and look in the alphabetical list for the disorder you need information for.