I want to take a step back from the details of particular human diseases – how they are transmitted, what they do when they infect, how they can be prevented – and talk about one of the broader goals of disease control, eradication.

Eradication of a disease in simple terms means completely removing a disease from all human populations, everywhere in the world. So far, only one human disease has been eradicated: smallpox, with the last case documented in Somalia in 1977 and eradication formally confirmed by the World Health Organisation in 1980. The only other successful eradication campaign was for rinderpest, a virus affecting cattle, in 2011. A number of other human viral and parasitic diseases are potential candidates for eradication, including dracunculiasis (Guinea worm) which is on the brink of eradication, polio, onchocerciasis (trachoma, or river blindness), measles, mumps, rubella, cysticercosis (pork tapeworm) and yaws.

Before we go any further, let me clarify the difference between two commonly interchanged terms – eradication and elimination.

Eradication, as explained above, means the complete removal of a disease from human populations throughout the world. Once a disease is eradicated, control measures such as vaccination and surveillance are not needed any more and the disease becomes an historical curiosity.

Elimination, however, is more limited in scope and means the removal of a disease from a particular region. When a disease is eliminated, control measures are still absolutely necessary. Without vaccination (if available), hygiene, protection of water supplies, education, accurate diagnosis, efficient surveillance and other control measures particular to the disease, an eliminated disease can be re-imported from other regions and completely reverse the elimination efforts. It is at the point when a disease is eliminated from multiple regions that eradication becomes possible, and also at that point when a final push is needed to chase down and completely get rid of every single case of the disease.

Disease eradication is obviously not easy. Many diseases will never be eradicated, no matter how good our vaccines, treatments and surveillance networks are. Influenza (the flu), for example, is transmitted amongst many different species of birds including chickens, ducks and other water birds, can also be found in pigs, and is highly variable. There is an International Task Force for Disease Eradication, which defines three criteria for determining whether a disease is a candidate for eradication:

1) Epidemiologic vulnerability

This criteria refers to the features of the disease, how it is spread and how it can be controlled. Does it hide in animal species, or is it a disease that only affects humans? Can it be easily distinguished from other infections and accurately diagnosed? How easily does it spread?

2) Availability of an effective intervention

Is there a vaccine against the disease? How effective is the vaccine? If there isn’t a vaccine, is there another easy way to stop people getting sick with this disease? How much does the intervention cost? Can it be used everywhere in the world – even in remote, hard to reach places without electricity?

3) Evidence of successful control

Is there a region where the disease has been eliminated? This proof can be a critical factor in deciding to attempt disease eradication.

Of course, even if a disease fulfils these criteria, other factors such as political will, cost, available resources and potential community benefit all need to be considered too. Armed conflict or natural disasters can derail disease control efforts. Even once eradication is set as a goal, progress can be sporadic. The campaign for measles eradication, for example, made big gains in the early 2000s, but has stalled in more recent years. Countries such as the US and Australia, where measles has been officially eliminated, are still vulnerable to outbreaks from cases imported from other countries. The biggest threat is not the anti-vaccination movements in places like Australia, but the strikingly high number of cases in countries where vaccination levels are low and the disease still runs rampant. The recent outbreak in the US involved 121 people, a figure that is dwarfed by the 89,000 measles cases in the Democratic Republic of the Congo in 2013, or the 107,000 people infected in China last year.

Disease eradication is an incredible goal. This year will almost definitely see Guinea worm added to the list of eradicated diseases. Whether other diseases follow we will have to wait and see, and hope that the perfect convergence of ability, motivation and resources can rid the world of more disease