The following text is for questions 42 to 47.

Leeches

Leeches are slimy worms which have two suckers. The big one is placed at the rear and the smaller one at the mouth. The leeches are able to expand their bodies since they have powerful muscles. Most leeches eat blood of other creatures. By piercing its skin, a leech attaches its body to the victim's skin and sucks the victim's blood. It secretes a material which prevents the blood from clotting. The leech body will expand from its normal size as it sucks the blood. It is hard to remove a leech, once it attaches itself to your skin. You should put salt on it so that the leech falls off and dies.

    There is one species of leech that sucks animals' blood by entering their breathing passages. Later, the breathing passage will be completely blocked and make the victim suffocate. 

    In the past, people believed that having too much blood inside the body may cause some diseases. Then doctors would attach leeches on the patient's skin to suck the patient's blood. In fact, some leeches were specially bred for this purpose. 

What is the last paragraph about?