13．Depression is not a choice．Rather，it is a true medical condition like high blood pressure or heart disease．In the United States alone，more than 17million people suffer from depression with women being twice as likely as men to suffer from the disease．Depression crosses racial and socioeconomic lines． Anyone can suffer from it，and those that do are not"crazy"or"weak"．They have a serious yet highly treatable disease．
Feeling down？You're not alone．If depression seems pretty common，it is．One in every ten adults suffers from depression．And that's what has scientists confused．If depression causes harm to the body，why is it so widespread in the population？Some scientists think they may have the answer．Depression may have an evolutionary （进化） benefit．It may improve one's chances of fighting off infection （感染）．
Scientists have known that depressed people show an immunoreactive （免疫反应性的） condition known as inflammation （炎症），even when they are not infected．For several years they have been finding that genes （基因） linked to depression also affect the function of the immune system．Stresses，which can lead to depression，also change the immune system．These findings suggested that depression was somehow linked to the body's ability to fight disease．
Some scientists now come up with the idea that depression might have had an evolutionary advantage in the days before modern medicine and antibiotics （抗生素）．Infection was a major cause of death in early human history．Surviving those infections determined which genes were passed to the next generation．
They doubt that depression helped people，especially children，stay alive from infection．Extreme tiredness leads to inactivity and less social contact which helps keep infection from spreading．Having no appetite helps avoid food sharing which can also spread infection．Sleeplessness，which is associated with both depression and immune system activation，may have been a mechanism to keep a person alert（机警） to fight off enemies after injury．
If scientists are correct about this link，it could lead to new treatments for depression．They are interested in drugs that treat conditions where the immune system attacks the person's own body，known as auto immune diseases．It is possible that medicine known to treat auto immune disease may be successful in treating depression also．
80．The underlined sentence in Paragraph 1meansD．
A．there is a depression line between different groups and societies
B．people at a higher social position may tend to suffer from depression
C．people who are living with depression could connect with each other
D．anyone can get depression whatever their economic states and backgrounds are
81．In Paragraph 2the authorA．
A．introduces an idea on depression
B．gives an example to show his idea
C．explains how depression works
D．describes the depression problems
82．According to the passage，which of the following is TRUE？B
A．Depression can spread infection．
B．Stress may affect the function of the immune system．
C．Women are less likely to suffer from depression．
D．Modern medicine is widely used for treating depression．
83．The writer mainly wants to tell people that depressionC．
A．is serious but highly treatable disease
B．could have its advantages in scientific research
C．may improve one's chances of fighting off infection
D．is a true medical condition like high blood pressure．
（A） 
