The United Nations estimates that more than one billion people worldwide do not have enough to eat. That's more than the combined populations of the United States, Canada and the European Union.

Hunger not only affects people's ability to work or study at school - it also kills. A child dies of hunger every six seconds, says the U.N. World Food Programme (WFP).

Malnutrition is not just a shortage of calories but also a lack of particular nutrients.

Deficiencies in iron, iodine, Vitamin A and zinc are ranked among the U.N. World Health Organisation's top causes of death from disease in developing countries worldwide.

A common myth is that people are hungry because there is not enough food available or because of over-population. However, it is almost always poverty that prevents people from buying what is readily available in their local markets, rather than food shortages.
Disasters such as drought, flooding, hurricanes and earthquakes can destroy crops and create food shortages but this does not automatically lead to an emergency.

For example, since independence India has been successful at preventing famines, even though the country has experienced many substantial crop failures, often covering large areas and sometimes causing sharp drops in the amount of food available nationally.

On the other hand, famines have occurred in Asia and Africa without a large drop in food production and availability. Sometimes a famine has even coincided with a peak in food production, as in the Bangladesh famine of 1974, in which an estimated 1.5 million people died.

Nobel prize-winning economist Amartya Sen argued that the Bangladesh famine was caused by a spike in food prices due to flooding coinciding with a fall in work opportunities for agricultural workers. That created a situation where many starved because they were unable to afford the food that was available.

Food emergencies usually result from years of stresses that gradually tip into crisis unless governments and aid agencies intervene.

It often takes several failed harvests before farmers run out of coping mechanisms, although the situation can be more sudden for people who lose their livelihoods in a particular region, with no means of finding work. This can happen when a sector fails, or when war forces people to flee their homes and land.

Often a food crisis affects only certain groups - not the entire population in a particular region.

Governments are extremely reluctant to call a food crisis a "famine", because it implies they have failed to stop a food shortage from turning into a major humanitarian crisis.

Similarly, aid agencies tend to avoid the f-word, either because they are dependent on the state for access to the vulnerable communities, or because it implies they too have failed in their response to the food shortages.

Different agencies have different definitions for what constitutes a crisis, an emergency or a famine.

International relief agency Medecins Sans Frontieres classes a food crisis as one person per 10,000 dying every day, a serious food crisis as two people per 10,000 dying in a day and a famine as five people per 10,000.

The U.S. Agency for International Development (USAID) says a famine is a catastrophic food crisis that results in widespread acute malnutrition and mass mortality. It is a process, rather than an event, with a beginning, a middle and an end.

See our life-cycle of a famine to find out more.

CAUSES OF FOOD SHORTAGES

North Korean cooperative farm workers spread fertiliser in a rice field. REUTERS/Gerald Bourke

Food shortages have a variety of causes - many of them interlinked. Many if not most are caused by a combination of the issues below.

POVERTY

Ultimately, the main reason people are unable to feed themselves is not that food is not available but that they cannot afford it.

But poverty also reduces food output. Africa produces only around 700 kg of maize per hectare against a possible 7 tonnes per hectare. Farmers often lack irrigation and Africa has the lowest fertiliser usage in the world - a measure of how its farmers are simply unable to afford the inputs used by their developed world counterparts.

NATURAL DISASTERS

The impact of natural disasters such as drought, flooding, hurricanes and earthquakes can vary according to people's levels of poverty. Droughts tend to hit the poorest subsistence farmers much harder than large commercial growers, who have access to better irrigation and more resilient seed types.

Climate change is having an increased impact on food production as droughts and flooding become more frequent and more severe.

Some experts say global warming may demolish people's coping strategies and push already vulnerable families over the edge. Shrinking access to fertile land and water may trigger refugee crises and conflicts.

CONFLICTS

Conflict can drive people from their homes and away from their normal food supply, leave them unable to afford food or simply stop them planting. In Sudan's Darfur region, where violence has uprooted 2.5 million people, normal agricultural output has been slashed.

Other people may lose their incomes and therefore be unable to afford food. Food supplies may be disrupted by fighting or deliberate blockade.

In many conflicts, warring parties are accused of blocking aid and commercial food deliveries particularly to areas seen supporting rebels - although this is against international humanitarian law.

In any case, war makes delivery of food much more difficult, particularly if supplies are looted. There is some evidence that malnutrition rates in Darfur rose as attacks on aid agencies increased, reducing deliveries.

GLOBAL FOOD PRICES

Rising global food prices affect people's ability to buy enough to feed their families. In 2007 and 2008, the global price of basics like rice, wheat and maize soared, triggering riots in many countries.

Not surprisingly, the hardest hit are the poorest - especially the urban poor, who spend as much as 80 percent of their income on food.

Countries most affected are those which need to import large amounts of food, often because of failed harvests. Many governments introduce food subsidies or export restrictions to counter rising costs, but some critics say these only exacerbate price rises on global markets.

The price rises were caused by a combination of increased demand from India and China for grain and meat, a rise in the use of biofuels produced from food crops, a spike in transport costs, and poor harvests.

DISEASE

Disease can drive food shortages in a variety of ways.

The HIV/AIDS pandemic in southern Africa is seen as contributing to food shortages both through killing farmers, destroying critical local knowledge and pushing families deeper into poverty - thereby often cutting their ability to grow and produce food.

Antiretroviral drugs, which contain the effects of HIV, are less effective if people are malnourished. They also produce much stronger hunger pangs, and some HIV-positive people are reported to have stopped taking the medication during food crises as the hunger pangs became too painful.

Some other disease outbreaks such as the 2005 Ebola-like Marburg virus in Angola put pressure on food supplies as deliveries ceased due to drivers staying away.

COMPLEX EMERGENCIES

In reality, almost all food crises contain a mixture of the above, with the different factors often combining to produce a more powerful and serious emergency. Two of the most famous famines of the last 50 years - Biafra in Nigeria in the late 1960s and Ethiopia in the 1980s - were caused as much by politics and war as lack of rain.

Hundreds of thousands of people died in the short-lived breakaway state of Biafra in southeastern Nigeria because food didn't reach them. And most observers agree that hundreds of thousands of Ethiopians died when the effects of a two-year drought were drastically compounded by government policies of collectivising farms and resettling ethnic groups seen as sympathetic to insurgents.

The food crises that hit southern Africa in 2003 and 2005 were blamed on what aid workers called a triple threat" - the combined impact of poor growing weather, high HIV rates and deepening poverty.

As family members became ill, they had to sell goods to keep going and were able to work the land less - a vicious cycle that has continued in countries like Lesotho and Swaziland for years.

COPING STRATEGIES

When the TV cameras pitch up during a food emergency, their images of people at feeding centres don't tell the full story.

The same people who walk to feeding centres have spent months, even years, living with food shortages and have exhausted all their coping strategies.

Early coping mechanisms include eating seeds and selling livestock, jewellery and furniture. Men migrate in search of work and communities fall back on their cereal banks.

Some take out loans, and most cut back on their food intake and eat more wild foods. Many families are forced to pull their children out of school and spend less on healthcare.

If the shortages continue for several years, families may be forced to sell their tools, home and land.

FOOD AID

An Ethiopian child is fed by her mother at a feeding centre. REUTERS/Eliana Aponte

Initial food aid is usually provided by the government, often from its own strategic stores. Aid agencies may already be working feeding the most vulnerable groups or may be invited in.

The world's largest food aid distributor is the U.N. World Food Programme, based in Rome.

Funded mainly by donor governments and in part by private donations, it distributes food by aircraft, truck and pack animal in hunger spots around the world. Actual distribution on the ground is usually done in conjunction with partner agencies, usually local or international aid groups.

WFP buys its food locally as much as possible to reduce transport costs and help boost the local economy. The United States is the largest donor to WFP, supplying more than half of global food aid. Under U.S. legislation almost all U.S. food aid must be bought in the United States and the majority shipped on U.S. vessels.

Food is sometimes distributed as part of "food for work" schemes or through school feeding programmes that are also intended to keep children in school. In some places, cash transfers may be used to give people the money to buy food from local markets, thereby encouraging local growers.

Some international aid agencies also distribute their own food, while donor governments sometimes organise food aid shipments directly to countries in need.

While most experts agree food aid is essential to save lives, some also say it distorts local markets and can lead to dependence.

See our briefing on the global food aid controversy.

To find out more about how food aid works, read our fact sheet.

EARLY WARNING

While some disasters such as floods or wars can create food shortages almost overnight, the onset of food crises is usually slower and much more predictable.

For example, in the southern African food crisis of 2005, which affected countries across southern Africa from Namibia to Zambia to Mozambique, rains initially failed in late February and early March but the crisis only reached worldwide media attention in September.

That is because initially people were still eating crops left over from the previous year or the new harvest and so did not begin to run out of food for several months. But by September, granaries were empty and the crisis really began to bite.

Aid agencies would much rather get funding in place well before a food crisis to avoid having to spend much-needed cash on therapeutic feeding for the nearly dead or airdrops, which are much more expensive than delivering by truck.

A lot of effort has therefore been put into developing early warning systems such as the Famine Early Warning Systems Network (FEWS NET), funded by USAID.

LONG-TERM SOLUTIONS

Most aid agencies including WFP say the long-term aim should be to reduce dependence on food aid both through sustainable agricultural development and reducing poverty.

Aid agency CARE International says its programmes in West Africa's Niger aimed at reducing poverty and building sustainable agriculture cost around $30 a person - half the price of providing food at the height of the country's 2005 food crisis.

CARE estimates that by 2020 the world will have spent more than $333 billion this century fighting emergencies in Africa but that just $266 billion, spent differently, could instead have halved hunger by 2015.

Increasingly, some governments and aid agencies say cash transfers to the neediest help keep them from falling into serious hunger and malnutrition.

See our briefing on cash transfers.

The debate on the best way of boosting agricultural output and avoiding food crises is never-ending, but experts say more resilient seed types, more appropriate crops, better irrigation and business planning all have a role to play.

Most of the world's worst famines in the last century took place not in Africa but Asia, many in China and India. But both countries have since largely avoided serious food crises - although not chronic malnutrition.

Experts attribute that to better planning and agricultural techniques as well as strong overall economic growth. But while India has avoided famine and produces ample food, it still has one of the highest populations of undernourished people in the world.

Experts are divided over the best ways of improving agriculture in Africa. Some say improved fertiliser and seeds - including genetically modified types - are the answer, while others favour lower tech solutions. Agricultural experts offer a string of specific farming techniques for specific regions aimed at reducing soil erosion or water loss.

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