Why does the passage advise emptying or covering anything that holds standing water?

Dengue is caused by a virus that passes to people through the bite of an infected female mosquito, mainly of the species Aedes aegypti. A mosquito picks up the virus by biting a person who already has it, and about 8 to 12 days later it can pass it on.

About half of the world's population now lives at risk of dengue, and there are an estimated 100 to 400 million infections every year. Most people who catch it have mild symptoms or none at all, and recover within one or two weeks. When symptoms do appear, usually 4 to 10 days after the bite, they include a high fever, a severe headache, pain behind the eyes, aching muscles and joints, nausea and a rash.

A small number of people develop severe dengue, which often needs care in hospital. Its warning signs include severe stomach pain, vomiting that does not stop, fast breathing, and bleeding from the gums or nose. People who catch dengue for a second time are at greater risk of the severe form.

There is no medicine that cures dengue; treatment eases the pain. Paracetamol is advised, while painkillers such as ibuprofen and aspirin should be avoided, because they can make bleeding more likely. The best protection is to avoid bites, with covering clothes, nets and repellents, and to empty or cover anything that holds standing water, where the mosquitoes breed.