The specific measure of antigenic distance between two strains of influenza were calculated as P epitope values by the method suggested by Muñoz, et al., [39] . The P epitope value was calculated as the number of mutations within an antibody antigenic site divided by the number of amino acids defining the site. It is assumed that an antigenic epitope which has the greatest percentage of mutations is dominant, because that epitope is influenced by the greatest selective pressure from the immune system. The P epitope distance is defined as the fractional change between the dominant antigenic epitopes of one strain compared to another. The P epitope Calculator [76, 77] was applied for H3 sequences. Residues in antigenic epitopes were collected from several references [9, 13, 39, 40, 48, [78] [79] [80] [81] [82] [83] 83, 84] . Can "presumed consent" justify the duty to treat infectious diseases? An analysis BACKGROUND: AIDS, SARS, and the recent epidemics of the avian-flu have all served to remind us the debate over the limits of the moral duty to care. It is important to first consider the question of whether or not the "duty to treat" might be subject to contextual constraints. The purpose of this study was to investigate the opinions and beliefs held by both physicians and dentists regarding the occupational risks of infectious diseases, and to analyze the argument that the notion of "presumed consent" on the part of professionals may be grounds for supporting the duty to treat. METHODS: For this cross-sectional survey, the study population was selected from among physicians and dentists in Ankara. All of the 373 participants were given a self-administered questionnaire. RESULTS: In total, 79.6% of the participants said that they either had some degree of knowledge about the risks when they chose their profession or that they learned of the risks later during their education and training. Of the participants, 5.2% said that they would not have chosen this profession if they had been informed of the risks. It was found that 57% of the participants believed that there is a standard level of risk, and 52% of the participants stated that certain diseases would exceed the level of acceptable risk unless specific protective measures were implemented. CONCLUSION: If we use the presumed consent argument to establish the duty of the HCW to provide care, we are confronted with problems ranging over the difficulty of choosing a profession autonomously, the constant level of uncertainty present in the medical profession, the near-impossibility of being able to evaluate retrospectively whether every individual was informed, and the seemingly inescapable problem that this practice would legitimize, and perhaps even foster, discrimination against patients with certain diseases. Our findings suggest that another problem can be added to the list: one-fifth of the participants in this study either lacked adequate knowledge of the occupational risks when they chose the medical profession or were not sufficiently informed of these risks during their faculty education and training. Furthermore, in terms of the moral duty to provide care, it seems that most HCWs are more concerned about the availability of protective measures than about whether they had been informed of a particular risk beforehand. For all these reasons, the presumed consent argument is not persuasive enough, and cannot be used to justify the duty to provide care. It is therefore more useful to emphasize justifications other than presumed consent when defining the duty of HCWs to provide care, such as the social contract between society and the medical profession and the fact that HCWs have a greater ability to provide medical aid. In the course of providing health care service, health care workers (HCWs) are continually exposed to many workrelated health risks. One of these risks is the exposure to infectious diseases. These diseases can include the flu, AIDS, tuberculosis, and hepatitis, and can be transmitted through physical contact, exposure to contaminated blood, or via the respiratory system. And, needless to say, such risks do indeed at times prove fatal. The consequences of occupational exposure to pathogens are not limited solely to bodily infections. Each year, thousands of HCWs are adversely affected by psychological trauma stemming from months of anxiously awaiting the results of serological tests, tests made necessary due to potential infection incidents. The anxiety experienced by HCWs is related to the perception of risk from the incident and the resulting infection that may occur, and by the worry of what the reactions of others might be, such as colleagues, family, and friends, all who have to be informed. During this uncertain waiting period HCWs will frequently experience intrusive thoughts, problems concentrating, difficulty sleeping, frequent loss of temper, and a decrease in sexual desire, which can act as a catalyst to exacerbate any pre-existing and unresolved emotional issues [1] . And if it turns out that the health care worker has indeed been infected by one of these contagions, the serious personal consequences to that health care worker can include the postponement of childbearing, damaged personal relationships, having to alter sexual practices, experiencing the side effects of prophylactic drugs, chronic disabilities, loss of employment, denial of worker compensation claims, possible need for a liver transplant, and premature death [2] . AIDS, SARS, and the recent epidemics of the avian-flu have all served to remind us of the occupational risks faced everyday by HCWs; the result being that the recent appearance of these diseases has forced this issue onto the common agenda and helped to spark renewed interest in the debate over the limits of the moral duty to treat. In seeking an answer to this question it is useful to have an understanding of the occupational risks faced by HCWs as well as an understanding of the attitudes of HCWs to these risks. For example, studies conducted in various countries have shown that, especially when there was a risk of being infected with AIDS, HCWs may refuse to treat a patient on the grounds that there is a risk of being infected by this patient [3] [4] [5] [6] [7] [8] [9] [10] [11] . And despite the fact that the hepatitis viruses are transmitted more easily than HIV, it is the fear of being infected with HIV that causes many HCWs to experience the greatest amount of stress and anxiety [12] . In a study which compared the relative risks of transmission of both HBV and HIV, the reasons for physicians' underlying fears of particular contagions were also inves-tigated and described. [13] . According to the study, people initially percieve the risk to be greater when there is a high likelihood of death involved with infection (as with HIV) even though there may be less risk of infection, as opposed to when there is a higher risk of infection but a lower risk of death involved with that infection (as with HBV). Additionally, since the likelihood of sexually transmitting HBV between heterosexual partners is less than that of transmitting HIV, the consequences of HBV infection are again percieved to be less severe than the consequences of HIV infection. In this way, the hazards posed by HBV infection conflict less with the obligation to protect family members from harm. It was also found to be important that there is less of a stigma attached to having HBV than there is to having HIV. And, finally, the fact that there is a vaccine for HBV infection, which is more than 90 percent effective (for vaccinated HCWs the risk of death from infection is reduced by a factor of nearly twenty), also was found to greatly influence the perceptions of the physicians.