Exploration of ethnic differences in DNA methylation patterns show that differences in methylation are largely driven by differences in cell composition of the blood samples methylation is measured from. This raises important questions about when and how differences in cell composition arise. One possibility is that they occur in response to early life infection and persist through life. This project investigates the potential link between early life infection (using H. pylori infection as a marker) and variation in cellular composition of blood samples collected at the same study timepoints.