London, England (CNN) -- It's World Health Day on Wednesday, which this year has a special focus on urbanization and health.
It's a theme the World Health Organization (WHO) hopes will highlight the effects urbanization has on the welfare of people who live and work in cities.
The campaign will run from 7-11 April with the theme "1000 cities, 1000 lives," and will look at how the urban environment affects both our collective and individual health.
Six out of 10 people will live in urban areas by 2030, says the WHO, which promotes World Health Day on 7 April each year to commemorate the founding of the organization 62 years ago.
A billion people worldwide live in urban slums and increasing urbanization has created many health-related problems, WHO research has found.
And the urban poor suffer disproportionately from a wide range of diseases, says the WHO.
The WHO says many of these issues can be addressed by increasing access to social and health services, a fairer distribution of income and increased educational opportunities.
Daniel Dorling, professor of human geography at Sheffield University in the UK, says that living in cities is a recent phenomenon for most of the world's population.
He told CNN: "It is not something that we were evolved to do. Most people's great grandparents, or even grandparents in poorer countries, lived in small villages."
Dorling, who is a member of the WHO's scientific resource group on Health Equity Analysis and Research, said most of the health problems in developed urban areas were related to mental health, while in developing countries it was still physical health.
"Social isolation is an issue in big cities in rich countries, whereas in developing countries it is overcrowding, infant mortality, diarrhea and other health issues," he said.
Dorling said two good examples of cities that have good health records are Tokyo in Japan and Havana in Cuba.
He said Tokyo was a good example of a rich capitalist city that has an excellent health record, partly because there was more equality and a smaller gap between the poorest and the richest.
Cuba on the other hand, despite being a developing country, had infant mortality rates and a life expectancy as good as the richest countries.
"Cuba exports doctors. Even we [the UK] don't do that," said Dorling.
Nigel Thrift, vice-chancellor of the university of Warwick in the UK and a leading human geographer and social scientist, told CNN: "The most obvious factor is inequality. Any way you look at it, poor health is intimately correlated with inequality. Poverty is the real killer."