the most serious health problem in the world used to be infectious diseases such as typhoid fever,influenza (flu),and bubonic plague. In the 14 th century,for example,an epidemic of bubonic plague killed roughly one-third of the population of Asia and about half of the population of Europe.A flu epidemic in 1918 killed millions of people around the world.Today, however,vaccines and antibiotics have prevented the spread of many diseases. In fact, in many parts of the world today, infectious diseases are no longer considered to be the most serious health problem.