Are rotavirus infections generally deadly in third world countries?

Yes. They can be.: Rotavirus causes around 500, 000 deaths annually in kids under the age of 5, almost all of which occur in 3rd world countries. It is spread by fecal oral route and causes copious diarrhea and vomiting which then causes severe dehydration and even death secondary to this dehydration. Clean water an sanitary conditions would prevent most deaths.

Potentially: Potentially for infants if they get seriously dehydrated.