Though Nepal is not any more unsafe than any other developing country, update your preventive inoculations. Injections against meningitis, tetanus, hepatitis B, typhoid, perhaps cholera are recommended. Just keep yourself safe from stray dogs and monkeys. As said, prevention is better than cure, it's highly recommended that you bring medicines for common illness like nausea, vomiting,cold and flu when you come to Nepal. Though there are many pharamacy shops in Kathmandu and Pokhara, it's worth carrying some basic medicinal stuffs like insect repellent, sunscreen, lip balm, eye drops, bandages and so on.
Malaria is under control in Nepal.
The risk of catching it is small and only in the southern plains. Mosquito netting and repellent are advised when there, especially during the summer and monsoon seasons. Also, as a preventive measure, take chloroquine pills starting two weeks before and six weeks after your visit to the plains. AIDS is a growing problem. Don't be misled into believing that unsafe sex is okay.