Environmental changes wrought by population movement, destruction of habitats and other factors may be behind a resurgence of infectious diseases, a United Nations study says.

A rise in cases of diseases such as malaria and dengue fever, and the recent crossover to humans of others such as the Nipah virus, are linked to a host of changes that create more favorable conditions for their spread, according to a report by the U.N. Environmental Programme (UNEP) issued on Monday.

Deforestation, unplanned urban sprawl, poor waste management, pollution, building of roads and dams and rising temperatures are among the aggravating factors.

Infectious diseases cause about 15 million deaths annually, or about a quarter of all fatalities, UNEP says. In Southeast Asia and Africa, they account for two-thirds of all deaths, with the majority of them children and young adults.

The environmental roots of the rise in infectious diseases is one of the “emerging challenges” listed in UNEP’s annual Global Environmental Outlook.

“What is good for the environment is good for health, and what is good for health is good for development,” UNEP health and environment expert Hiremagalur Gopalan told a news conference.

The often fatal Nipah virus, normally found in Asian fruit bats, is believed to have crossed over to humans as the bats lost their habitats through forest fires in Sumatra and the clearance of land for palm plantations.

As the bats searched for fruit, they were brought into contact with pigs, which in turn passed the disease to their human handlers in the late 1990s, the report says.

Dengue fever, which was present in only nine countries in the 1970s, is now found in more than 100, most likely the result of increasing urban populations, the report says.

Since much urban growth occurs without planned sanitation, water treatment and sewerage, increased exposure to mosquitoes, rodents and other vermin provides more opportunities for diseases such as malaria, dengue, tuberculosis and hantavirus.

Mining, the damming of rivers and increased irrigation for agriculture also give mosquitoes more standing water in which to breed, the report says.

In the United States, cases of the tick-borne Lyme disease in New York and Connecticut have surged as humans have moved into forested areas where the deer that carry the ticks thrive, the report says.