Theoretical risks, such as the danger of people treading on syringes washed up on the beach, are distractions from the common routes of transmission and common dangers. In no other illness is there such constant displacement of attention from the real routes of transmission onto freak modes of transmission or minimal risks. The apparent abandonment of common sense in many people's assessment of the risks of HIV transmission suggests that fears of death, contamination and sexuality are being triggered. As if to confirm this, many people who understand the routes of HIV transmission very well nevertheless report irrational fears about HIV transmission, often in circumstances where they come into contact with people who have AIDS.

Many `theoretical' risks emerge as sources of anxiety as a result of uncertainties about the length of time which HIV can survive outside the body. It is important to put these uncertainties in context.

Although HIV may be able to survive outside the body for several days, this has only been proved by collecting blood from needles and syringes, or else in laboratory conditions. Whilst small quantities of HIV may indeed be able to survive in syringes or needles for several days, if we correlate this finding with the evidence of the unlikelihood of transmission from needlestick injuries (in which large quantities of blood are often transferred without infection occurring), we can see why this evidence is not sufficient to justify fears about theoretical modes of transmission.

In fact, infection through blood has only occurred from needlestick injuries and blood spillages either where blood has splashed directly from the body of an infected person into open wounds or where blood stored for transfusion or laboratory research purposes has splashed into open wounds or mucous membranes such as the mouth or eyes.

Another reason why these theoretical routes are unlikely is that heat, light, cold, water and chemicals are all likely to damage the virus. Thus virus–laden syringes in the ocean or HIV in sewage are impossible routes of transmission on this score too.

Theoretical risks may also emerge as a source of anxiety because of doubts about the quantity of HIV needed to infect a person. Although virologists argue that it may only take one virus particle to infect a person, it is important to bear in mind that four factors have to be fulfilled if HIV transmission is to take place (see The mechanisms of HIV transmission). All `theoretical' modes of transmission are very unlikely to fulfil these four factors.

Finally, theoretical routes of transmission may be given undue attention because of habitual public health concerns about the chances of the transmission of any infection by such a route. A good example of this is the repetition of fears about the transmission of HIV through food handling. This has been used to justify discrimination against people with HIV and gay men in catering on the 'theoretical' assumption that if someone cut his or her finger and bled into food while it is being prepared, and this food was then eaten by someone, it would constitute a risk.

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