Right now, yellow fever and meningococcus are the only two required vaccinations before traveling internationally. The first of the two, yellow fever, is required by International Health Regulations before entering certain countries in sub-Sahara Africa and tropical South America. A menigococcus vaccination is required by the Saudi Arabian government, but only during the Hajj (the annual pilgrimage to Mecca, Saudi Arabia). Yellow fever travels via infected mosquitoes, and menigococcus is spread through saliva.
While not required, it is strongly recommended to get vaccinations for both hepatitis A and B before traveling to almost any international destination. Transmission of hepatitis A can occur through direct contact between people and through exposure to contaminated goods, such as water, ice, shellfish, or fruits and vegetables and is found in most of the developing world. Hepatitis B is primarily transferred through contact with blood or blood-derived fluids and the vaccine should be taken if there is a chance of blood transfusions or sexual contact with an infected individual.
There is an estimated 22 million people affected by typhoid every year, it is a good idea to get vaccinated for typhoid before traveling. Travelers to South Asia have the greatest risk of contacting this disease, but it is also common in East and Southeast Asia, Africa, the Caribbean, and Central and South America. Typhoid is a disease known only to humans and is primarily transfered by water or food contaminated with feces.
Rabies is a serious disease that results in death and is found on every continent except for Antarctica. It is almost transmitted by an animal bite, usually from an infected dog or bat, but can also include monkeys. It is endemic (prevalent) to parts of Africa, Asia, and Central and South America.
