Although parasite fitness is often assumed to be equivalent to between-host transmission, such a simple algorithm overlooks the fact that transmission is a consequence of processes acting on at least two different scales, i.e., within and between hosts. Studying disease evolution therefore requires an understanding of the interactions between parasites, host resources, and host immunity at the within-host level; of parasite transmission, host demography, and epidemiology at the between-host level; and of the links between levels. Recent theoretical approaches have taken a more holistic view of disease evolution by explicitly tracking dynamics at both scales in a single unified framework – so-called ‘nested’ models. Such models require good resolution on the mechanistic details of within-host dynamics, which we’re working on.
When that mechanistic understanding is not available, one can use alternative approaches inspired by evolutionary quantitative genetics. Combining these tools with experimental data, we aim to understand how changing the within-host environment (either in subtle ways due to, for example, diet or through step changes such as those induced by interventions) experienced by parasites affects the evolution of disease life history.
Relevant papers:
Mideo, N., Acosta-Serrano, A., Aebischer, T., Brown, M.J.F., Fenton, A., Friman, V.-P., Restif, O., Reece, S.E., Webster, J.P. & Brown, S.P. (2013) Life in cells, hosts, and vectors: parasite evolution across scales. Infection, Genetics, & Evolution, 13: 344-347. PDF
Mideo N., Nelson W.A., Reece S.E., Bell A.S., Read A.F., & Day T. (2011) Bridging scales in the evolution of infectious disease life histories: Application. Evolution, 65: 3298-3310. PDFAPPENDICES
Day T., Alizon S., & Mideo N. (2011) Bridging scales in the evolution of infectious disease life histories: Theory. Evolution,65: 3448-3461.PDF
Mideo N., Alizon S., & Day T. (2008) Linking within- and between-host dynamics in the evolutionary epidemiology of infectious diseases. Trends in Ecology and Evolution 23: 511-517.PDF