Dengue fever is caused by the dengue virus, which is transmitted by mosquitoes. There are four types of the virus. Infection causes flu-like symptoms but can develop into severe dengue hemorrhagic fever in some cases. While most cases resolve on their own, severe cases involve bleeding, low platelet counts, and shock and can be fatal if not properly treated. Recurrent infection with a different virus type increases the risk of more severe disease. Prevention relies on controlling mosquito populations and avoiding bites.