Figure 1. Histology data and model of placental infection.

(a) The proportion of histology samples in each gravidity category in each study, which were classified as acute, chronic and past infections. The number of samples in each category is displayed below each gravidity category label in parentheses. (b) When placental sequestration occurs this results in an ‘acute infection', the infection then passes through the remaining stages of placental infection according to the stage-specific rates of progression until, if the pregnancy has yet to reach delivery, all evidence of sequestration is fully cleared. (c) When infection within the placenta consists of a combination of a recent acute infection and pigment from a previous infection, the observed histology depends upon whether the earlier infection has left behind ‘substantial pigment' (with probability P) in which case the overall result would be defined as ‘chronic' infection. Otherwise the result would be classed as ‘acute' infection.