There are the required, recommended, and if-you-so wish vaccinations. However, our advice, you rather be safe than sorry. And while many people are against vaccinations and medicine (which I understand), taking chances in the outback is a higher risk game. Ideally, vaccinations should be 4-6 weeks prior to your trip, but please discuss this all all other relevant information with our doctor.

Resources
Please be sure to check out the USA CDC for the most up to date information!

All Travelers
Make sure you are up to date on your routine vaccinations. This includes: measles-mumps-rubella (MMR) vaccine, diphtheria-pertussis-tetanus vaccine, varicella (chickenpox) vaccine, and your yearly flu shot.

Most Travelers
Hepatitis A and Typhoid are recommended for most travelers, as both can be received through contaminated food or water. Typhoid is especially recommended for people that will be traveling in more remote areas.

Some Travelers
Nepal has no risk of yellow fever, but does require proof of vaccination if you are coming from country with possible infections. This does not include the USA.
Other vaccinations to consider are: Hepatitis B (received through sexual contact, blood, etc.), Japanese Encephalitis (recommended for longer trips in rural areas), Malaria, and Rabies.