Individual factors are biologic or behavioral characteristics
associated with vulnerability to acquire or transmit
illness or infection [9,10]. These risks should be measured
when there is biological or public health plausibility
of being actual risk factors, ideally secondary to a
rigorous systematic review with meta-analysis. While objective
approaches to the synthesis of evidence for all
levels of risk are preferred, in 2012, there is generally
only sufficient levels of evidence for systematic reviews
and meta-analysis of individual-level risk factors given
the focus on this level of risk in the majority of epidemiological
assessments of HIV.