Gaza and Haiti are on the brink of famine, experts say. Here's what that means.

In Gaza, virtually every resident is struggling to get enough food and 1.1 million people — half the population — are expected to face the highest level of severe hunger in coming weeks, according to a report from the Integrated Food Security Phase Classification, an agency that monitors hunger globally. On Monday, the group warned that famine could occur in Gaza any time between mid-March and May without an end to hostilities and immediate access to essential supplies and services.

In Haiti, about 1.4 million people are on the verge of famine and more than 4 million need help accessing food, aid groups say.

But what does it mean for a region to fall into famine? And how could it happen in these places so fast? Here's what you need to know:

What is famine?

The IPC, a group of 15 global organizations and charities, was developed in 2004 during the famine in Somalia. The group uses a five-tier scale to monitor access to food and levels of hunger.

Famine is the top tier, Phase 5, "the absolute inaccessibility of food to an entire population or subgroup of a population, potentially causing death in the short term."

It occurs when 20% of households have an extreme lack of food, 30% of children suffer from acute malnutrition and at least two adults or four children per every 10,000 people die daily because of outright starvation or the interaction of malnutrition and disease.

That top level follows the Phase 3 "crisis" and Phase 4 "emergency" levels of food need. Worldwide, nearly 158 million people face crisis hunger situations or worse, according to the IPC.

While the depth of the food crises in both places is new, the underlying conditions are not, said Tobias Stillman, director of technical services and innovation at the aid group Action Against Hunger, via email.

Even before the war, 80% of Gazans relied on humanitarian aid and nearly half of all households didn't have enough food, he said. In Haiti, millions were already coping with emergency levels of hunger and crisis levels of food need.

"When families and entire nations live so close to the brink, it is all too easy for conflict or other shocks to push them into catastrophe," Stillman said.

What are hunger, malnutrition and starvation?

Hunger is the informal term for the feeling that occurs "when our bodies need or expect food," Stillman said. Aid groups say hunger occurs when people can't afford or physically obtain sufficient nutrition for an extended period of time.

Malnutrition is a medical condition that occurs when people don't get the right calories to grow and function properly, leading to health problems. The deadliest form of malnutrition is severe acute malnutrition, which occurs when children are too thin for their height.

"This can happen suddenly, caused by a severe hunger crisis, or it can occur over time," Stillman said.

Starvation is not a technical term, but it describes extreme suffering or death caused by lack of food.

Death from starvation can come "surprisingly quickly," Stillman said. Without food, the body uses carbohydrates and fats first, then turns to breaking down protein, including muscle and vital organs. The body begins to shut down functions, including digestion, which make it harder to absorb any nutrients that are available. People suffer from extreme fatigue and become listless as the body tries to conserve energy.

Without specialized treatment, organs stop functioning and the body's defenses can't fight infection. Many times, people without food die of common infections. If that doesn't happen, vital organs shut down and the heart stops.

Who is most vulnerable?

Children younger than 5, pregnant and breastfeeding women, the elderly and people with underlying health conditions are most at risk from malnutrition. In acute crises like that seen in Gaza, malnutrition affects the youngest children first, experts said.

What happens if famine is declared?

A declaration of famine would be made by top United Nations officials based on the IPC criteria. Such a declaration carries no binding obligations on U.N. members or states, but serves to focus global attention on the problem.