A mosquito’s spit can be worse than its bite alone. In fact, a new study finds that in some cases, the insect’s saliva makes the viral disease dengue fever more severe. In mice, scientists found that mosquito spit weakened blood vessels, making them more permeable, or “leaky.” Immunologist Michael Schmid and colleagues report in the online June 16th issue of PLOS Pathogens that easier exchange between the blood and tissues may help the virus spread faster — and increase the severity of disease. Dengue virus enters the bloodstreams of nearly 400 million people a year. It's delivered through the sharp proboscises of tropical Aedes mosquitoes, which also deliver a spit-load of other molecules as they slurp a meal. There are four strains of dengue, which can cause bone and muscle aches, high fever and, in severe cases, death. Overcoming one type of dengue doesn’t protect the host from the other three strains. In fact, subsequent infections are often worse.