good physicians in yelahanka What causes dengue fever? Dengue fever is caused by one of four dengue viruses. When a mosquito infected with the dengue virus bites you, the virus can enter your blood and make copies of itself. The virus itself and your immune system’s response can make you feel sick. The virus can destroy parts of your blood that form clots and give structure to your blood vessels. This, along with certain chemicals that your immune system creates, can make your blood leak out of your vessels and cause internal bleeding. This leads to the life-threatening symptoms of severe dengue.