On Jan.11, Mariam Soraya Vorster, a 33-year-old American tour guide working in the northern Thai city of Chiang Mai, became sick with what was thought to be food poisoning and died.

Vorster's death would be the first of five in Chiang Mai in just six weeks, four alone in a hotel called the Down Town Inn. Two more women staying in the same hotel became severely ill and nearly died.

Two of the dead -- British pensioners George and Eileen Everitt died within hours of each other. Authorities told a press conference that the two, 78 years old and 73 respectively, had died of heart attacks within minutes of each other although their son, Stephen Everitt, told the New Zealand Herald that both were active, healthy and had no history of heart problems.

The authorities' version of the deaths has yet to be openly and vigorously challenged by the victims' embassies, although no doubt discreet enquiries are continuing. This seeming lack of action by US, British and New Zealand diplomats may reflect a case-hardened response given the large numbers of foreign nationals who die from one cause or another in Thailand each year.

These are hardly the first unexplained deaths in Thailand. Routinely in Bangkok, especially around the seamier districts, supposedly healthy young males are diagnosed in autopsies as having died of heart attacks."

In fact, although there is no public data on foreign fatalities and detailed descriptions of the cause of death of tourists in Thailand, anecdotal evidence indicates that a disturbing number of foreign nationals die of unexplained causes – as many as 50 on the island of Phuket alone over a recent eight-month period. That has disturbing implications for the country's tourism industry, which earned Thailand more than US$19 billion from 15.8 million foreign visitors and accounted for around 7 percent of GDP in 2010.

According to Prime Minister Abhisit, speaking in early March, revenues from tourism could account for 11 percent of GDP by 2020 if visitor numbers double as forecast to around 30 million. These huge numbers, and the present and future importance of tourism to Thailand and key players in the industry, has had a direct impact on how the authorities deal with events that may damage the country's reputation as an attractive destination for overseas visitors.

In Phuket's case, a report by police between the end of December 2009 and mid-August 2010 on the island itself, which was expanded upon by the local Teak Door blog, offers a snapshot of how the deaths occurred over the period.

Three deaths were acknowledged by the police to be result of direct criminal violence. However, a further 15 were recorded as dying from heart failure, falls and 'unknown causes' – as broad category that would also encompass deaths from drug overdoses. Another five were recorded as suicides, and the remainder died as a result of traffic accidents, drowning or electrocution. This pattern is likely to be repeated in Thailand's other beach resorts, with variations in inland areas such as Bangkok and Chiang Mai.

On Koh Phi Phi in 2009*, for instance, a supposedly completely healthy 26 year-old American woman died of unknown causes while staying in a guest house. Her boyfriend was admitted to a nearby hospital, vomiting severely. The guest house's air conditioning was suspected, the victims' families said. The next day, two Norwegian women developed the same symptoms. One of the two, 22 years old, died in a Phuket hospital later. Her friend was seriously ill but survived.

Unsurprisingly there is a great deal of speculation over the causes of the Down Town Inn cluster of deaths in Chiang Mai. With food poisoning ruled out, and no indication that anyone unconnected with the hotel has either died or became seriously ill exhibiting similar symptoms during the period, other causes are being examined.

These range from toxic spider bites to deliberate poisoning – perhaps intended to damage the Down Town Inn's reputation as part of a personal or business dispute. Local political leaders, the police and medical services have contributed to this atmosphere through their failure to provide credible and independently audited explanations for the deaths.

This case will continue to grow unless it is resolved quickly and to the satisfaction of the victims' families and the media. Although it is unlikely to harm Thailand's overall appeal as a tourist destination it will do little shake off the country's reputation of a being a great place to holiday, but with dark and dangerous forces barely below the surface.

* Corrected Mar. 13, 2011

Gavin M. Greenwood is a security consultant with the Hong Kong-based Allan & Associates firm.