Once a person gets the dengue virus from a mosquito, the virus attaches to and enters the person's white blood cells. (The white blood cells are part of the immune system, which defends the body by fighting off threats, like infections.) As the white blood cells move around the body, the virus makes copies of itself. The white blood cells react by making many special proteins, like interferon, which tell the immune system to work harder because there is a threat in the body. These proteins cause the fever, flu-like symptoms, and severe pains that happen with dengue.