完形填空Expectancy

　　A hundred years ago，life expectancy in developed countries was about 47： in the early 21st century， men in the United States and the United Kingdom can expect to live to about 74. Women to about 80， and these ages are rising all the time. What has brought about these changes？ When we look at the life span of people l00 years ago， we need to look at the greatest killers of the time. In the early 20th century， these were the acute and often highly infectious diseases such as smallpox. Many children died very young from these diseases and others， and the weak and elderly were always at risk.

　　In the developed world these diseases are far 1ess deadly today， and in some cases have almost disappeared. A number of factors have led to this： improvements in sanitation and hygiene， the discovery and use of antibiotics， which make bacterial diseases much less dangerous， and vaccinations against common diseases. In addition ， peoples general health has improved with improvements in our general environment： cleaner air， better means of preserving food，better and warmer housing，and better understanding of nutrition.

　　Genetically，we should all be able to live to about 85 but while people do live longer today， there are still some big killers around that are preventing US from consistently reaching that age. The problems that affect people today are the more chronic illnesses， such as heart disease and strokes， and those spread by viruses， such as influenza and AIDS l. Of course， cancer is a huge killer as well. In most cases these diseases affect older people， but there are worrying trends in the developed world with problems such as obesity leading to more heart disease and illnesses such as diabetes at younger ages.

　　The killers today can be classed as "lifestyle diseases"，which means that it may be possible to halt their progress.