You can get aids from HIV, which is through sex. There is no known cure. You get Malaria from mosquito's and there is a cure.
Answer:
You didn't give us enough information but I'm going to guess Spain since they did send out "missions."
Spanish colonies needed many workers because they needed to grow their crops, mine gold and silver, and work in their new cities. Spain wasn't able to have enough Spanish workers so they ended up taking Indians as their slaves, giving them very little food and water forcing thousands to die of starvation and overwork.