The immune system in our bodies ①fights the bacteria and viruses which
cause diseases. Therefore, whether or not we are likely to get various diseases
②depend on how well our immune system works. Biologists used to ③think
that the immune system was a separate, independent part of our body, but
recently they ④have found that our brain can affect our immune system. This
discovery indicates that there may be a connection between emotional factors
and illness.