The World Health Organization reports half of the world's population lives in cities, with about one billion people living in urban slums. To mark this year's World Health Day, the U.N. agency is launching a campaign to highlight the need for good urban planning to make cities healthy and safe.

The World Health Organization reports for the first time in history, more people are now living in urban than in rural areas.  It says by the middle of this century, seven out of every 10 people will live in a city.

WHO Director-General Margaret Chan says the most explosive growth is expected in Asia and Africa.  Unless mega-cities in these regions are well planned and efficiently managed, she warns the inhabitants will be susceptible to multiple health threats.

"From inadequate sanitation and refuse collection to pollution and accidents from congested traffic, from children playing barefoot in soil or water contaminated by untreated waste, to outbreaks of infectious diseases that thrive on filth and crowded conditions," she said.  

The World Health Organization says many cities face a triple threat of infectious diseases, chronic non-communicable diseases, and risks from road accidents, injuries, violence and crime.

Dr. Chan says cities tend to promote unhealthy lifestyles, such as fast-food restaurants, smoking, and the harmful use of alcohol and other substances. "These lifestyle changes are directly linked to obesity and the rise of chronic diseases like heart disease, stroke, some cancers, and diabetes.  And, these conditions are costly to treat, not only to the individuals, but, also to the society and their families.  And they are increasingly concentrated in the urban poor," she said.    

Dr. Chan notes even the poorest cities are able to enact measures that can improve their environments.  She cites Kigali, the capital of Rwanda as one city where tremendous progress has been made.

"I was there a couple of months ago.  I was really struck and impressed with how clean it is.  And, of course, I look into the background.  I talk to the people.  The president of that government set himself up as an example.  One Saturday of every month, he goes out walking around picking up garbage, rubbish and put them away properly," said Dr. Chan.  

The World Health Organization says cities must be planned, managed and governed well.  It says developing countries that follow this prescription can raise life expectancy to 75 years or more, but warns badly governed countries can expect life expectancy as low as 35 years.