On this World Health Day, the focus is on the increasing danger of vector-borne diseases, which kill more than one million people each year.

According to the World Health Organization (WHO), more than half of the world’s population is at risk for diseases spread by ticks, mosquitoes, flies, and other blood-sucking creatures (“vectors”) that transmit infectious microbes while feeding on people. Each year, more than one billion people are infected with a vector-borne disease, such as malaria, dengue fever, Lyme disease, and yellow fever.

April 7 is World Health Day 2014, and WHO is raising awareness of these risks with the slogan “Small bite, big threat“—and emphasizing that these illnesses are entirely preventable.

“A global health agenda that gives higher priority to vector control could save many lives and avert much suffering. Simple, cost-effective interventions like insecticide-treated bed nets and indoor spraying have already saved millions of lives,” said Dr. Margaret Chan, WHO’s director general, in a statement. “No one in the 21st century should die from the bite of a mosquito, a sandfly, a black fly, or a tick.”

These vector-borne diseases tend to affect the world’s poorest populations, especially in places where there is inadequate access to secure housing, safe drinking water, and sanitation. People suffering from malnutrition and people with weakened immune systems are especially susceptible to many of these illnesses.

According to WHO, schistosomiasis, which is transmitted by water snails, is the most widespread of all vector-borne diseases. It affects almost 240 million people around the world. Children who live near unsanitary, infested water are particularly vulnerable to this disease, which causes anemia and cognitive or learning delays. Schistosomiasis can be controlled through regular mass treatment of at-risk groups with a safe, effective medicine, as well as by improving access to safe drinking water and sanitation.

Within the past 20 years, many other vector-borne diseases have re-emerged or spread to new parts of the world. Environmental changes, a huge rise in international travel and trade, changes in agricultural practices, and rapid global urbanization are causing an increase in the number and range of many vectors and putting new groups of people—for instance, travelers (for business or pleasure)—at risk.

Dengue fever, for example, which is spread by mosquitoes, is now found in 100 countries, putting more than 40 percent of the world’s population at risk. It has recently been reported in Portugal and Florida.

Meanwhile, Greece is reporting its first cases of malaria in 40 years. In its World Health Day press release, WHO said that these outbreaks highlight the need for continued vigilance.

“Vector control remains the most important tool in preventing outbreaks of vector-borne diseases,” said Dr. Lorenzo Savioli, director of WHO’s Department of Control of Neglected Tropical Diseases. “Increased funds and political commitment are needed to sustain existing vector-control tools, as well as medicines and diagnostic tools—and to conduct urgently needed research.”

On World Health Day 2014, WHO is calling for a renewed focus on vector control and better provision of safe water, sanitation, and hygiene.