A Civil Aid Services trainer who died of a rare tropical disease could have been weakened by an uncommon underlying condition, an expert witness said yesterday. Cheng Ching-kwong could have had a deficiency in his immune system that made him more susceptible to the disease and could have contributed to an unusual complication that killed him, Lai Kang-yiu told an inquest into Cheng's death. Cheng, 40, died in May last year after a scratch from an insect bite on his arm was infected with Chromobacterium violaceum. An autopsy report said he died of the infection, which was complicated by refractory septiceamic shock and multiple organ failure. His blood, lungs, liver and bone marrow were all found to be infected. Dr Lai, clinical director of the intensive care unit at Queen Elizabeth Hospital, said the rapid deterioration could be attributed to an unusual complication induced by an overreacting immune system. Cheng's body might have produced too many infection-fighting elements called cytokines, which could have attacked his own immune system and killed his white blood cells. The loss of white blood cells could cause organ failure. He said Cheng died of 'a rare complication of a wound infection by an unusual organism that is usually associated with a high mortality rate'. 'The patient may possibly have an underlying host defect that may predispose this patient to the susceptibility to this unusual organism and rare complications,' Dr Lai said. Cheng's wife, Tai Sui-fan, has told the inquest that her pleas for the doctors at Prince of Wales Hospital - where Cheng was admitted on May 26 last year - to look at the infected wound were disregarded and if they had been heeded the appropriate antibiotic would have been given much earlier. Yesterday the attending doctor in the hospital's intensive care unit, Choi Yuk-sang, said doctors did not find out until the day after Cheng's admission that he was infected by the rare disease. 'No one in the intensive care unit or the hospital would have thought of this bacterium because it was just too rare,' Dr Choi said, adding he had not heard of it before. He said he had already used an antibiotic that could cover unknown bacterial infections including anthrax and Chromobacterium violaceum, but Cheng did not respond to it. Cheng developed an abscess on his arm after staying at a Tuen Mun camp site for a cadet training on May 21 last year. The inquest continues today.