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Viruses and bacteria have complex interactions with their hosts, beyond mere replication in them. They range from those that are detrimental, to others that may be non-pathogenic or even beneficial. Molecular techniques can help to unravel these interactions, sometimes revealing phenomena that benefit host as well as microbial populations.

Amplification methods could be used to develop tests so that very small protein amounts could be detected. A piece of DNA--a DNA tag--could be coupled to an antibody, and this could then be used to detect a protein antigen by amplification of the DNA tag.

It is important to establish the likely route of infection for all reported HIV infected individuals if the HIV epidemic is to be understood. Investigating routes of infection may bring unusual infection routes to light. Steps have been taken in the United Kingdom (UK) to establish the likely mode of HIV acquisition for everyone reported as infected. If an initial report is incomplete the clinician caring for the patient is asked for relevant information. If further information is needed, and an interview is feasible and acceptable to both the clinician and the patient, information is collected from the patient through face-to-face semistructured interview. Such follow up has identified 19 cases (among 34,000 records of individuals with HIV and/or AIDS for which probable routes of infection have established) who seem to have acquired HIV infection in unusual circumstances. Seven of the 19 cases described in this paper are thought to have acquired HIV infection in the UK, two in Spain, and ten in countries with a high prevalence of HIV infection. This paper describes the circumstances in which HIV transmission is believed to have occurred.