The mortality rates of European settlers per 1,000 mean strength in the 19th century are provided by Appendix Table A2 of Acemoglu, Daron, Simon Johnson, and James A. Robinson (2001) "The Colonial Origins of Comparative Development: An Empirical Investigation," American Economic Review, 91(5), pp.1369-1401.
See the unpublished appendix (on Johnson's website) for the construction of this dataset.
It's also shown to be correlated with volatility of growth and economic crises. See Acemoglu, Johnson, Robinson, and Thaicharoen (2003).
An "improved" version constructed by David Albouy (see under the heading of "Papers" on his website.)
For criticism on this improved dataset, see Acemoglu et al. (2005)