Dengue infection is a leading cause of death and sickness in tropical and subtropical parts of the world. Dengue is caused by one of four viruses that are transmitted by the bite of an infected mosquito. Dengue hemorrhagic fever is a more serious form of dengue infection. Primary symptoms of dengue appear three to 15 days after the mosquito bite and include high fever and severe headache, with severe pain behind the eyes that is apparent when trying to move the eyes. Other associated symptoms are joint pain, muscle and bone pain, rash, and mild bleeding. Many affected people complain of low back pain. The lymph nodes of the neck and groin may be swollen. Young children and people infected for the first time typically have milder symptoms than older children and adults. Dengue hemorrhagic fever starts with the typical signs and symptoms of dengue as described above. The fever lasts from two to seven days. After the fever begins to abate, symptoms occur that are related to increased permeability of the capillary blood vessels. These symptoms can include severe abdominal pain, prolonged vomiting, and breathing problems. Bleeding tendencies, including easy bruising, nosebleeds, bleeding gums, skin hemorrhages, and even internal bleeding may occur. The disease may progress to failure of the circulatory system, leading to shock and death.