Fast growing cities, home to more than half the world's population, is the World Health Organisation's new target for a major campaign to counter a triple threat to health.
The global campaign starting on World Health Day aims to rally 1,000 cities to "open up public spaces to health", by closing off portions of streets to traffic, to encourage exercise in parks and clean up campaigns.
One of the WHO officials behind the drive, Lori Sloate, said it was important to forge a global movement for action in cities, "while there's still time because we've just passed the tipping point."
The world's urban population passed 3.0 billion in 2007, exceeding the rural population for the first time, according to the United Nations. By 2030, 60 percent of the world's growing population is expected to live in cities.
Cities are home to a "triple threat" to health, Sloate told AFP.
"Infectious diseases is one, particularly in places where there's lack of water and sanitation," she explained.
Stressful urban lifestyles filled by fast food, smoking and alcohol also fostered chronic diseases such as diabetes, cancers and cardiovascular problems in congested cities.
"This can be exacerbated by the lack of physical activity for example, by increased respiratory problems through air pollution," said Sloate.
"Then finally the third is also linked more specifically to the cities in the sense that they're concentrated there: it can be violence, crime, road traffic and injuries."
The challenges are also amplified by urban poverty, with an estimated 830 million people who currently live in slums, according to the UN's HABITAT agency.
"The idea .... was to mobilize the efforts of cities and to focus on the importance of municipal leadership in adressing health problems," she explained.
UN humanitarian chief John Holmes has also warned that some cities housing millions of people in coastal areas would be threatened by rising sea levels with climate change, or in earthquake zones.
"The risks of megadisasters in some of these megacities are rising all the time," Holmes warned, predicting a lot more deaths in future natural disasters.
The biggest megacities include Tokyo, with a population of more than 35 million, and Mumbai, New Delhi, Mexico City and Sao Paulo with more than 20 million inhabitants each.
The WHO campaign is backed by a website http://1000cities.who.int with social networking links to encourage community action.