Over the past decade, global health has become a priority in U.S. foreign policy, and U.S. appropriations for global health-related efforts have more than tripled. Neglected tropical diseases (NTDs) have become an important part of these efforts. Congressional appropriations for NTDs have grown from $15 million in FY2006 to $100 million in FY2014. The Administration requested $86.5 million to support NTD programs in FY2015. Heightened congressional interest in combating NTDs has been reflected not only in higher appropriation levels but also in the development of caucuses on these issues. In October 2009, the House Malaria Caucus expanded its purview to include NTDs, and in September 2012, the Senate Malaria Caucus did the same. Tropical diseases encompass all diseases that occur solely, or principally, in the tropics. Of these, the World Health Organization (WHO) describes 17 as "neglected," since resources for curing, controlling, and researching improved treatments for these were limited until recently. The 17 NTDs are found mostly among the poorest people in 149 countries and territories ( Figure 1 ), primarily where access to clean water, sanitation, and health services is limited. Some NTDs are transmitted by people; others are spread by vectors like snails, flies, or mosquitoes; and several others proliferate in soil or water. Among the 17 NTDs, 7 account for roughly 90% of the global NTD burden ( Table 1 ). These are the three soil-transmitted helminths (intestinal worms), schistosomiasis (snail fever), lymphatic filariasis (elephantiasis), trachoma, and onchocerciasis (river blindness). Intestinal worms, or STH, account for roughly 80% of the seven most common NTDs ( Figure 2 ). NTD control efforts are grouped into two categories: those that require individual treatment and those that can be addressed by mass drug administration (MDA). Several NTDs can be addressed through MDA, when an entire community with known cases is treated irrespective of individual disease status. Of the 17 NTDs, 8 can be treated with MDA. These include the seven most common NTDs as well as foodborne trematode infections. Although the seven most common NTDs can be treated with MDA, complex transmission cycles complicate efforts to eliminate or eradicate them. STH, for example, is contracted through contact with or ingestion of worm eggs that lie in soil. The eggs are deposited in the soil through fecal matter and can remain there for several years. Access to adequate sanitation facilities is a critical component of interrupting the STH transmission cycle, as the STH medicines kill the adult worms, but not the eggs. Due to the limitations of drugs, WHO recommends a five-pronged strategy: mass drug administration innovative and intensified disease-management; vector control and pesticide management; safe-drinking water, basic sanitation and hygiene services, and education; and veterinary public-health services. Since WHO coined the phrase "neglected tropical diseases" in 2003, global efforts to address these ailments have accelerated. In 2007, WHO released the Global Plan to Combat Neglected Tropical Diseases , which outlined several goals and targets to reach by 2015 for global control, elimination, and eradication of NTDs ( Figure 3 ). In 2012, WHO released a report, widely known as the Roadmap , which highlighted progress in reaching the 2015 goals and established new ones for 2020. One year later, it released a report outlining progress in achieving the 2020 goals, as described in the Roadmap . According to the reports, guinea worm disease has been nearly eliminated and was endemic in only four countries by 2012: Chad, Ethiopia, Mali, and South Sudan. Nearly 97% of the 542 cases identified in 2012 occurred in South Sudan, where instability remains a key threat to eradication. Since the 2012 report was released, WHO has noted further progress in addressing guinea worm disease, with 148 cases reported in 2013 and only 10 cases reported from January 1, through April 30, 2014. WHO has also since noted progress in other areas. Annual sleeping sickness cases, for example, fell by more than 70% from 1995 through 2012 ( Figure 4 ). In January 2012, representatives from endemic countries, the private sector, and donor nations convened in London, England, to affirm their commitment to combatting NTDs. Participants signed the London Declaration on Neglected Tropical Diseases , which highlighted the role each signatory would play in reaching the 2020 goals outlined in the Roadmap . Appendix A lists donor commitments. These pledges included the following: $785 million (includes preexisting commitments) to strengthen drug distribution and program implementation; 1.4 billion drug treatments annually; access to drug compound libraries to identify new treatments; increased financial support for NTD programs (some of which amended previous commitments to ongoing efforts), such as: commitments by the governments of Bangladesh, Brazil, Mozambique, and Tanzania to devote political and financial resources to combat endemic NTDs. Annual donations of NTD treatments have continued to grow ( Figure 5 ). In 2013, donors provided nearly 1.35 billion NTD treatments, up from 995 million in 2011. Nonetheless, these treatments are expected to reach only 36% of those who need them, with some 1.4 billion people lacking access. Treatment access rates vary per disease and region. For example, roughly 37% of children with STH in need of deworming received treatment in 2012 ( Figure 6 ). Regional treatment rates ranged from 7% in the Middle East to 47% in Southeast Asia. Pharmaceutical companies have donated sufficient supplies of medicines for most NTDs, yet a $1.4 billion funding gap persists. Health analysts assert that an additional $200 million is needed annually between 2014 and 2020 to deliver the donated drugs and meet other programming costs. The $1.4 billion does not include additional resources needed for research and development (R&D) of new treatments, vaccines, and testing supplies. NTD experts maintain R&D is vital. Some observers are troubled by emerging evidence that hookworm, one of the STHs (that comprise 80% of all NTDs), may be developing resistance to existing medication. In addition, the international community cannot meet 2020 goals with existing tools ( Figure 7 ). There is an urgent need for additional drug treatments and vaccines for those NTDs that are difficult to treat, are costly to manage, and can have severe clinical outcomes if left untreated. These are Buruli ulcer, Chagas disease, human African trypanosomiasis, the leishmaniases, leprosy and yaws. Given the complexity of these diseases, patients must be seen at well-equipped health facilities by well-trained, specialized technicians; these types of facilities may not be available in many of the affected countries. Additionally, exposed populations need to be systematically screened because these diseases are generally asymptomatic during the periods when treatments would be most effective and carry the least dangerous side-effects. In addition to the above-mentioned diseases, dengue has become a growing global health problem. Dengue is the leading cause of serious illness and death among children in some Asian and Latin American countries. More than 2.5 billion people are now at risk of infection, close to 40% of the world's population. Of these, between 50 million and 100 million contract the disease annually. Before 1970, only nine countries had experienced severe dengue epidemics. The disease is now endemic in more than 100 countries. Dengue has been detected in a number of U.S. states and territories, including Florida, Texas, and Puerto Rico. There is no specific treatment or vaccine for dengue, which can lead to death if left untreated. Early detection and proper care can reduce fatality rates from about 20% to less than 1%. Congress has taken steps to encourage drug development for certain diseases; some NTDs are among them. Two pieces of legislation established a framework for incentivizing drugs for rare diseases or disorders: The Orphan Drug Act of 1983, P.L. 97-414 , followed by the Rare Diseases Act of 2002, P.L. 107-280 . These acts defined rare diseases or disorders as those that affect fewer than 200,000 individuals in the United States. This definition includes NTDs and other rare or orphan conditions. Through these programs, 26 drugs received a new drug approval (NDA) or a biologic license application in 2012. The FDA Amendments Act of 2007 ( P.L. 110-85 ) created a priority review voucher (PRV) to incentivize the development of drugs and biologic products for tropical diseases. The applicant would receive a voucher at the time of approval of certain tropical disease products that "offer major advances in treatment where no adequate therapy exists." This voucher would allow the applicant a priority review of another drug product. This voucher could be transferred (sold) to one other sponsor. The draft guidance issued by the FDA describes the diseases and conditions covered by the PRV. The act also permits the Secretary to designate other diseases by regulation, which could include "any infectious disease for which there is no significant market in developed nations and that disproportionately affects poor and marginalized populations.... " The list includes tropical diseases beyond the 17 NTDs, namely cholera, malaria, and tuberculosis (TB). At the same time, the act excludes some NTDs listed by WHO, including rabies, cysticercosis/taeniasis, and Chagas disease. A variety of U.S.-based institutions support global efforts to control NTDs. These institutions include the federal government, pharmaceutical companies, philanthropic organizations, and NGOs.