Dengue happens when a person is bitten by an infected female mosquito of the Aedes species, primarily Aedes aegypti or Aedes albopictus. These mosquitoes become infected when they bite a person who already carries the dengue virus in their blood. After the mosquito ingests the virus, it replicates inside the mosquito's body, eventually reaching the salivary glands. When the infected mosquito bites another person, it transmits the virus through its saliva into the person's bloodstream, initiating infection

. The virus infects skin cells and immune cells near the bite site, then spreads to white blood cells and throughout the body, causing symptoms like fever, rash, and severe pain. The infection can range from mild to severe, with complications including bleeding and shock in severe cases

. Dengue can also be transmitted from a pregnant woman to her fetus, and rarely through blood transfusions, organ transplants, or needlestick injuries. There is some evidence suggesting possible transmission through sexual contact, but the primary and most common mode remains mosquito bites

. In summary, dengue occurs primarily through the bite of infected Aedes mosquitoes that have previously fed on a person carrying the dengue virus, leading to the spread of the virus to new human hosts