The U.S. State Department has issued a warning to Americans against travel to parts of Paraguay because of an outbreak of dengue fever.

An employee of Uruguayan Ministry of Health fumigates a park during a campaign against Dengue fever in Montevideo, 15 Mar 2007

A statement issued Tuesday said citizens may want to particularly avoid the area of Asuncion, Paraguay, where most of the nation's infections are concentrated. At least 18,000 people in Paraguay have been infected, and at least a dozen of those have died of the severe form of the disease, known as dengue hemorrhagic fever.

Some experts say the number of cases is significantly higher than the official reports. Paraguayan officials have been accused of a responding inadequately to the problem.

The mosquito-transmitted disease causes high fever, headaches, and pain in muscles and joints. No vaccine exists. It cannot be spread directly from person-to-person.

Experts say a hot, wet, South American summer, good breeding conditions for mosquitos, has contributed to the rise of the disease. Officials are urging people to eliminate any standing water on their property and use fumigation to kill mosquitos on their property.

Bolivia and Brazil have also suffered recent outbreaks, and Uruguay recently reported its first confirmed case.