The legionella bacteria is more prevalent in Hong Kong than most people think and two illnesses it causes go underdiagnosed, a microbiologist says. The bacteria thrives in warm, stagnant fresh water of up to 45 degrees Celsius, is resistant to low levels of chlorine and is known to multiply in dirty cooling towers, water storage tanks and whirlpool spas, said Ho Pak-leung, president of the University of Hong Kong's Centre of Infection. The organism can even be found in old shower heads. A person can become exposed to the bacteria by, for example, accidentally breathing in legionella-contaminated water vapour. It cannot be transmitted from person to person, however. If exposed, a person, depending on their immune system, can either come down with Pontiac fever, which mimics a mild flu, or a lung infection, a pneumonia known as legionnaires' disease, said Dr Ho, who is also a microbiology consultant at Queen Mary Hospital in Pok Fu Lam, and a former member of Hong Kong's Prevention of Legionnaires' Disease Committee, which issues guidelines for dealing with the disease in the city. David Cunliffe, principal water quality adviser with South Australia's Department of Health, said generally 'the organism is far more common than the disease'. 'The risk from legionella is relatively low but you can reduce it even further by taking commonsense measures - basically keeping things clean,' he said. Additionally, antibiotics could combat the most serious form of infection if identified early. Symptoms for Pontiac fever could develop several hours after exposure, while legionnaires' disease would begin to manifest several days after exposure, Dr Ho said. When most patients consulted a doctor with symptoms 'no test will be performed', he said. A person with Pontiac fever may not even go to the doctor, while a patient complaining of chest problems may just be prescribed antibiotics. 'But from a public health perspective, it is important to know because it allows you to find out whether we need to do something with different water sources, like spa pools ... cooling towers and [other] water storage systems,' Dr Ho said. 'Unless you have a good surveillance system, you cannot make a proper risk assessment ... outbreaks could occur without us knowing.' John Herbert, managing director of Kelcroft, a Hong Kong-based built-environment consultancy, agrees with Dr Ho. Legionella-based illnesses are 'under-diagnosed everywhere. It's a global problem - not just a Hong Kong problem', he said. He had been on many Hong Kong rooftops as part of his job and he summed up the state of many water-cooling towers in one word: atrocious.