SARSについての文章

次の英文を読みなさい。

SARS is probably a long-established infection that may have been circulating quite naturally among wild life or small animals in southern China. It's quite possible that some form of respiratory disease resembling SARS has been around for many years, but a change in the sort of agricultural activity has perhaps nurtured and allowed the emergence of a slight variant, a much more dangerous variant that can probably spread to human populations much easier than some of the past infections have done. With the more intense farming of caged chickens, with ducks close-by, and also in some cases of pigs, there has been a tremendous amount of mixing of respiratory viruses. Historically this is the way new epidemics of another respiratory disease - influenza - have appeared in South China. When pigs, chickens and ducks are kept close to people living in high density, there are plenty of targets for new mutant influenza viruses emerging from these carrier animals. The same is probably true of the SARS virus. These new viruses swap back and forth between the different animals to humans, mutating with destructive effect. Once our immune system kills off one new virus, another stronger one emerges, to which there is no defense. Some form of the SARS virus may have been around for hundreds of years. There are a number of different ways in which you can catch a virus from an animal. One is if you are butchering the animal, you could infect yourself at that time by a small cut. You can have food that's directly polluted with animal feces, you can have respiratory spread, or many other ways. SARS is not the first new deadly infection to originate from animals. The most recent, examples of viruses crossing species would be AIDS and hepatitis B, both of which seem to have come from apes or chimpanzees, and so crossed over that species' barrier.[2004 Waseda]

学部受験レベルの平易な文で、解説もいらないとも思いますが、後ほど若干注をつけます。