The UN noted that about 2 billion people do not consume a sufficient amount of vitamins and minerals. In India, the second-most populous country in the world, 30 million people have been added to the ranks of the hungry since the mid-1990s and 46% of children are underweight. Famines have been frequent in world history. Some have killed millions and substantially diminished the population of a large area. The most common causes have been drought and war, but the greatest famines in history were caused by economic policy. In Afghanistan, about 35% of households are food insecure. The prevalence of under-weight, stunting, and wasting in children under 5 years of age is also very high. The persistence of wet markets has been described as "critical for ensuring urban food security", particularly in Chinese cities. The influence of wet markets on urban food security include food pricing and physical accessibility. Food insecurity has been an issue for Mexico throughout its history. Although food availability is not the issue, severe deficiencies in the accessibility of food contributes to the insecurity. Between 2003 and 2005, the total Mexican food supply was well above the sufficient to meet the requirements of the Mexican population, averaging 3,270 kilocalories per daily capita, higher than the minimum requirements of 1,850 kilocalories per daily capita. However, at least 10 percent of the population in every Mexican state suffers from inadequate food access. In nine states, 25–35 percent live in food-insecure households. More than 10 percent of the populations of seven Mexican states fall into the category of Serious Food Insecurity. The issue of food inaccessibility is magnified by chronic child malnutrition, as well as obesity in children, adolescents, and family. Mexico is vulnerable to drought, which can further cripple agriculture. In 2019, Singapore managed to produce only 13% of leafy vegetables, 24% of its eggs, and 9% of its fish. In 1965, it was still able to produce 60% of its vegetable demand, 80% of its poultry and 100% of its eggs. In 2019, it announced it launched the "30 by 30" program which aims to drastically reduce its food insecurity through hydroponic farms and aquaculture farms. The Agriculture Department defines food insecurity as "limited or uncertain availability of nutritionally adequate and safe foods or limited or uncertain ability to acquire acceptable foods in socially acceptable ways." Food security is defined by the USDA as "access by all people at all times to enough food for an active, healthy life." National Food Security Surveys are the main survey tool used by the USDA to measure food security in the United States.