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The most serious health problem in the world used to be infectious(传染的)diseases such as typhoid fever, flu, bubonic plague(黑死病). In the 14^th century, for example, bubonic plague killed roughly one-third of the population of Asia and about half of the population of Europe. A flu epidemic（流行病） in 1918 killed millions of people around the world. Today, however, vaccines and antibiotics have prevented the spread of many serious diseases. In fact, in many parts of the world today, infectious diseases are no longer considered to be the most serious health problem. In the United States, the number one health problem is the new epidemic of obesity.
What does it mean to be obese? Most doctors and researchers use the body mass index（指数）, or BMI, to determine whether someone is overweight or obese. The BMI equals a person’s weight in kilograms divided by the square of the person’s height in meters. A normal BMI is between 18.5 and 24.9. a person with a BMI of 25.0 to 29.9 is considered to be overweight, while a person with a BMI of 30 or above is said to be obese.
What is causing the epidemic of obesity? One obvious contributing factor is the sedentary lifestyle that many people in the 21^st century are leading. Getting around by car and working at a desk don’t demand much physical activity.
A second cause of obesity epidemic is an increase in the availability and consumption of junk food. The Centers for Disease Control and Prevention defines junk foods as “foods which provide calories primarily through fats or added sugars and have minimal amounts of vitamins and minerals.” Studies have shown that people are consuming a lot more calories daily than they used to, and they are consuming many of those calories outside of regular meals.
The popularity of fast-food restaurants over the past 30 years has also contributed to be the obesity epidemic. The goal of most fast-food restaurants is to make high profits by producing meals cheaply and efficiently, with little concern for the nutritional content of food. As a result, much of the food served in fast-food restaurants is high in saturated fats, sodium, and sugar. The “great taste” of the food comes from artificial flavorings(调味品) created in science laboratories. For example, roughly 50 artificial ingredients are used to make strawberry milkshake sold in one popular fast-food restaurant. High-quality meat has been replaced by low-quality meat that is full of preservatives to make it taste good. While the nutritional content of much fast food is declining, the serving sizes have been getting larger, encouraging people to consume more calories.