The viruses that cause Zika, chikungunya, and dengue are an ever-present concern in Nicaragua so use of DEET mosquito repellent and other mosquito bite prevention strategies(PDF from the CDC) are important and highly effective safety precautions. Because of the presence of Zika, women who might be pregnant and partners of pregnant women should know that the CDC advises against their travel to Nicaragua.
Malaria is present but rare in our partner communities and the CDC (as of 2018) has no recommendations for these areas other than mosquito avoidance. If you will be traveling to the eastern half of Nicaragua before or after your JustHope trip, you should know that the CDC's recommends chemoprophylaxis for those areas.
JustHope's experience has been that diligent use of DEET mosquito repellent and other bite prevention strategies (long sleeves/pants, netting, permethrin-treated clothing, etc.) have proven to be effective protection against these viruses for our participants.
Regarding DEET mosquito repellent, we do not recommend 100% DEET, as that is such a high concentration that it is easily over-applied to no greater benefit. Repellents with under 50% DEET are ideal. More information about DEET is available from the CDC.