[decimon]

I checked Wikipedia, and it says millions died of typhus in Russia during WW I and the subsequent Russian civil war. Russians are certainly not immune.

If they're human then they are not immune. But in this campaign they were possibly not louse infected as were the French.

[hellbender]

I'm having trouble researching the issue with an internet connection slower than those found on the Russian steppes, but I think surviving typhus confers lifetime immunity. More of the Russians may have had the disease already before the war. Also, as has been said, the French forces were larger and more concentrated.

I also wondered why the French were affected more by yellow fever than the Haitian blacks. This is probably the answer:

" Disease outbreaks in towns and non-native people may be more serious because of higher densities of mosquito vectors and higher population densities.[15]"

Although the pathogens are very different, typhus is also an insect-borne infection, so the same factors might apply as with yellow fever.