More than 1.5 billion people - including the world's poorest - are affected by one or more of a group of so-called neglected tropical diseases. The diseases are called neglected because, compared with malaria for example, they receive little funding or attention from national governments and medical and donor communities despite the vast numbers of people affected.
They thrive where people live in extreme poverty with poor sanitation and little access to healthcare - usually in remote rural areas, urban slums or conflict zones - and disproportionately affect women and children. Many have been eliminated everywhere except among the most marginalised, and are therefore often forgotten.
A few of the diseases - including rabies and anthrax - kill relatively quickly, but the majority infect people for several years and are more likely to maim than kill. These include river blindness, sleeping sickness, elephantiasis, chagas disease, guinea-worm disease and leprosy.
Most are caused by parasites which are spread by mosquitoes, snails and tsetse flies. Some spread in contaminated water and soil. In many cases they go undetected until it is too late to prevent disability or death. To make matters worse, people disfigured by disease often face discrimination and stigma.
There are cheap treatments available for several of them (inlcuding yaws, roundworm, hookworm and whipworm infections, elephantiasis, river blindness, bilharzia, and trachoma), and pharmaceutical companies have donated large numbers of doses for free. But new treatments need to be developed in case drug resistance occurs, health experts say.
Some diseases still have no specific treatment, including dengue which affects millions every year. Others have only expensive treatments that are difficult to manage where early treatment is vital to avoid disability or death.
The main challenge with producing new drugs is the cost. Pharmaceutical companies spend millions of dollars to find and test a new drug and there is little financial incentive in producing new drugs for people who cannot afford to buy them.
Many ideas for treatments developed in research institutions are never tested. This is because by far the most expensive stage of drug development is the clinical trials, and usually only pharmaceutical companies have the means to carry them out. They take about six years to complete and involve thousands of volunteers. Another problem is that many developing countries do not have the capacity - either the training, staff or regulatory laws - to carry out trials. So the drugs cannot be tested on people who are most exposed to the diseases.
The good news is that there is growing awareness of the problem internationally, and an increased focus on eliminating the diseases.
In 2012, the World Health Organization published a roadmap that set ambitious targets to curb many of them. These targets were endorsed by donor governments, leaders of countries where the diseases are endemic, pharmaceutical companies and NGOs in the London Declaration 2012.
Successes include the reduced prevalence of elephantiasis after widespread treatment, and China and South Korea have eliminated it. Leprosy used to affect millions of people and cases now number in the tens of thousands. Guinea-worm disease has dropped dramatically from 3.5 million cases in 1986 to just 126 in 2014. There have also been successes with schistosomiasis, Chagas disease, river blindness and trachoma.
WHO lists 17 neglected diseases, although there are regional and national variations: dengue, rabies, trachoma, Buruli ulcer disease, endemic treponematoses, leprosy, chagas disease (American trypanosomiasis), human African trypanosomiasis, leishmaniasis, cysticercosis, dracunculiasis (guinea-worm disease), echinococcosis, foodborne trematode infections, lymphatic filariasis (elephantiasis), onchocerciasis (river blindness) schistosomiasis (bilharziasis) and soil transmitted helminthiasis (roundworm, hookworm and whipworm infections).