Mosquitoes!
Millions of people worldwide die of infections from these tiny insects, every year.
My Father got a Malaria during WWII & suffered from it for life from then on. He was stationed in Port Moresby.
For work, have had to stay in TPNG & PNG, many times. Was 'religious' about taking my Malaria Tablets six weeks before travel, whilst there, & six weeks after being there. I was determined NOT to get Malaria!