CCR5 and smallpox: Galvani & Slatkin considered the possibility that either plague or smallpox may been responsible for increased frequencies of a specific deletion in the CC-chemokine receptor type 5 gene (CCR5-Δ32) observed in European populations: the smallpox-like pathogen could increase the frequency of CCR5-Δ32 to a frequency of 10%, but this required sustained selection over 680 years [45].