Under the scorching sun and through heady humidity, Salem bin Shabz and his team of health workers move slowly through the narrow alleyways of Al-Koud, a densely populated slum in Yemen’s port city of Mukalla.

Every time they spot a pool of stagnant water, they stop and inspect the puddle. Armed with dippers and sample containers, they are on the hunt for mosquito breeding grounds amid an outbreak of the deadly dengue fever.

Makalla has become one of Yemen’s worst-hit areas for dengue fever, a viral infection spread by mosquito bites that can cause fevers, headaches, body aches, nausea, and in severe cases, death.

Bin Shabz kneels beside an open drainage chamber where a puddle of stagnant water, discharged from a household air conditioner, has turned green after standing for weeks.