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Epidemic foci of Plasmodium knowlesi malaria have been identified during the past ten years in Malaysia, in particular in the States of Sarawak and Sabah (Malaysia Borneo), and in the Pahang region (peninsular Malaysia). Based on a review of the available recent international literature, the authors underline the importance of molecular biology examinations, polymerase chain reactions (PCR), performed with primers specific for P. knowlesi, since the current microscopic examination (haemoscope) may fail to distinguish P. knowlesi from Plasmodium malariae, due to the very similar appearance of the two parasites. P. knowlesi has been described as the causal agent of life-threatening and lethal forms of malaria: its clinical picture is more severe when compared with that of P. malariae, since the disease is characterized by greater parasitaemia, as opposed to that documented in the course of P. malariae disease. The most effective carrier is Anopheles leucosphyrus: this mosquito is attracted by both humans and monkeys. Among primates, the natural hosts of P. knowlesi are Macaca fascicularis and Macaca nemestina, while Saimiri scirea and Macaca mulatta, which cannot become infected in nature, may be useful in experimental models. When underlining the potentially severe evolution, we note the key role played by prompt disease recognition, which is expected to be more straightforward in patients monitored in endemic countries at high risk, but should be carefully implemented for subjects being admitted to hospital in Western countries suffering from the typical signs and symptoms of malaria, after travelling in South-East Asia where they were engaged in excursions in the tropical forest (trekking, and similar outdoor activities). In these cases, the diagnosis should be prompt, and suitable treatment should follow. According to data in the literature, in non-severe cases chloroquine proves very effective against P. knowlesi, achieving the disappearance of signs and symptoms in 96% of cases after only 24 hours after treatment start. In the light of the emerging epidemiological data, P. knowlesi should be added to Plasmodium vivax, Plasmodium ovale, P. malariae, and Plasmodium falciparum, as the fifth aetiological agent of malaria. During the next few years, it will become mandatory to plan an appropriate surveillance program of the epidemiological evolution, paying also great attention to the clinical features of patients affected by P. knowlesi malaria, which are expected to worsen according to the time elapsed; some studies seem to point out greater severity according to increased parasitaemia, paralleling the increased interhuman infectious passages of the plasmodium.

The beginning of the third millennium has been characterized by the emerging and progressive characterization of a novel malaria Plasmodium pathogen of simian origin (Plasmodium knowlesi), which now represents the fifth human malaria parasite. Evolutionary, environmental, and diagnostic-clinical features are briefly outlined on the ground of the most recent literature evidences.

During the last months of 2003, outbreaks of avian flu caused by influenza A (H5N1) virus were described in eight Asian countries, and has paralleled an epidemic of human influenza caused by the same avian virus A (H5N1). Between December 26, 2003 and October 13, 2005, there had been 117 documented human infections and 60 deaths (mortality, 51%) in four Asian countries: Vietnam, Thailand, Cambodia and Indonesia. Human influenza sustained by H5N1 virus is usually characterized by a severe lower respiratory tract infection, leading to a progressive respiratory failure and several extra-respiratory complications. Infected birds have been the primary source of influenza H5N1 infection in humans in Asia, but transmission from poultry to humans is very limited at present, and requires a direct exposure to live birds, whereas there was no significant risk related to eating well-cooked poultry meat. Human-to-human transmission of H5N1 virus has been suggested in several household clusters, but it seems very uncommon at present. All the viral genes of human H5N1 isolates were of avian influenza origin, which indicates absence of reassortment with human influenza viruses, but H5N1 continue to evolve and persistent monitoring is requested to identify any increase in viral adaptation to man, which could lead to a human pandemic.