Bats are impressive creatures. They are known to carry dangerous diseases like SARS without getting sick. So researchers wanted to know: How do bats fight off so many deadly diseases? Learning their secret could help doctors better treat humans who have these diseases, therefore saving millions of lives. To solve this problem, a team of researchers from Australia and China analyzed bat DNA. The scientists studied two kinds of bats--the Australian megabat, known as the flying fox, and the Chinese microbat. The scientists then arranged the bat genome   in order by comparing their DNA. The researchers now think that the special immunity   to illness that bats have may be connected to their ability to fly. Bats are the only mammals   that can truly fly. Flying uses up a lot of energy. The energy burned for flying produces something in the animal called free radicals  . Free radicals can damage DNA and lead to diseases such as cancer. However, bats are not affected by free radicals produced during flight. The reason is that bats carry special genes to repair any damaged DNA. Scientists think that because bats fly and produce free radicals, they've evolved to carry genes that shaped their special immune system. "We're thinking that the evolution of flight led to a sort of effect, influencing not only the immune system but also things like aging and cancer," says Chris Cowled, of the Australian Animal Health Laboratory. "We think we've really found something special." Researchers believe that this discovery might one day be key to helping humans fight diseases such as cancer that attack the immune system and damage DNA. According to researchers, bats can fight off many deadly diseases mainly because   _  .
they have developed special immune system through evolution they are able to fly, which burns a lot of energy free radicals in their bodies can damage DNA and cause diseases they carry no special genes to repair damaged DNA