Brazil has the highest incidence of dengue fever in the world.

Last year, cases spiralled to a new high of 1.4 million and the trend shows no signs of abating. São Paulo, host city for the opening ceremony of the FIFA World Cup on June 12, battled a major dengue outbreak this month. "Our campaign has not been effective," laments Guilherme Ribeiro, an epidemiologist who specialises in infectious diseases at the Institute of Collective Health at the Federal University of Bahia. "We spend a lot of money and dengue is a priority for the government but unfortunately 2013 was the worst year on record."

Ribeiro understands the mosquito-borne disease well having suffered the fever twice and studied it up close in his native Salvador. A sprawling metropolis of 2.6 million with high rises towering over a sea of shanty towns, Brazil's third largest city is a prime breeding ground for the Aedes aegypti mosquito which transmits dengue. Salvador is one of the 12 host cities for the World Cup and health officials are paying close attention. In mid-June, the rainy season will start tapering off and dengue will be at its peak in the region.

Unlike malaria and yellow fever which are mainly found in the Amazon states and other thinly populated parts of the country, dengue is rampant in urban areas where 85% of Brazil's population lives. ; Rapidly growing cities and towns have provided an ideal environment for the spread of dengue. A tropical climate combined with poor infrastructure for water and sanitation encourage the proliferation of the Aedes mosquito, which can breed in as little as a teaspoon of water.

But what makes the urban mosquito especially dangerous is the fact that in densely populated neighbourhoods, it does not have to travel far to infect a large number of people. A recent study warned that, "overcrowded cities present highly favourable conditions for increased transmission," and that São Paulo was in danger of becoming a hot spot because all four dengue virus strains were in circulation there during an outbreak in 2013 – a first for a big city in Brazil.

There is no cure for dengue and it can occasionally be fatal. Public health experts are pinning their hopes on a vaccine which they believe will be the most effective way to combat the disease. Several vaccines are in development and Brazilian scientists are working on one version at the Butantan Institute in São Paulo. It was developed in partnership with the US-based National Institutes of Health. Early results are promising and the vaccine will be moving into phase three clinical trials at the end of next year.

In the meantime, the best way to get rid of mosquitoes is to eliminate their breeding areas but Salvador is a perfect example of how chaotic urban sprawl can make the task virtually impossible. "The mosquito is extremely well adapted to our urban environment," says Ribeiro pointing to the blue water tanks which are a common sight on rooftops around the city. They also happen to be an ideal incubator for mosquito larvae which thrive in dark, damp spaces. The tanks are widely used because piped water supply throughout the city is intermittent. There is widespread public awareness about the need to keep tanks and water containers covered and clean but it does not always translate into action. "The campaign only works when the community participates," says Angélica Andrade Santos, a health official in the Pau da Lima area.

One of the favelas (shanty towns) in Pau da Lima is a base for epidemiologists like Ribeiro who go there to conduct field studies on infectious diseases. There are already a few dengue cases in late March, including one 16-year old with hemorrhagic fever, the most virulent kind. According to a yet-to-be published study conducted in 2010 by the Institute of Collective Health and the Oswaldo Cruz Foundation, 95% of the adults in this favela have been exposed to the virus. But dengue is not confined to poor neighbourhoods. Other studies have found that the virus circulated in densely populated neighbourhoods across the city affecting rich and poor alike.

Dengue Mata (Dengue Kills) billboards greet visitors as they drive into the city from the airport. It is a blunt warning from the health department to the many visitors and soccer fans who will soon be arriving in Salvador. Simon Hay, an Oxford-based epidemiologist, urged Brazilian authorities to aggressively eliminate mosquitoes around the northeastern stadiums.

But Ribeiro says it is unlikely that the ultra-modern Arena Fonte Nova poses a dengue risk because it is a well maintained public space with very little standing water. He advises visitors to instead be alert to the threat on the streets saying, "one week in Salvador is more risky than being at the stadium."

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