Carriers We may think of an infectious disease as a process, a conflict 

 between two species. The invader attacks with a small army, which grad- 

 ually increases in numbers as the parasite lives at the expense of the host. 

 The beginnings are therefore mild, and for a time there is no indication that 

 the host is being injured. When fever and other "symptoms" appear, the 

 host has already begun to react. If antibodies are produced rapidly, the host 

 recovers. Sometimes, however, the host recovers without completely routing 

 the invader. The parasite adapts itself to the chemical conditions of the host, 

 and the host tolerates the parasite: neither appears to be injured. But the 

 germs being discharged from the body are just as virulent when they invade 

 another host. That makes the "carrier" a possible danger to other persons. 



The first typhoid carrier on record in the United States was Mary 

 Mallon, to whom seven outbreaks of typhoid fever were traced over a period 

 of years, by 1907. Later 30 other cases were traced to her directly, making 

 a total of 56 cases, of whom three died. She was kept under observation or 

 in confinement for over thirty years, until her death in 1939. As many as 

 400 typhoid carriers have been under control at one time in New York 

 State. Diphtheria carriers are also watched in a similar way. In such cases 

 the "dangerous" person is perfectly innocent of all wrongdoing; yet he has 

 to be regulated in his activities and movements for the protection of others. 



In Brief 



Most plants and many of the lower animals can regenerate parts that are 

 injured or destroyed. 



Among the higher animals cut and damaged tissues are replaced with 

 scar tissues. 



Injured cells apparently give out substances that stimulate the growth of 

 new tissue. 



Some poisons stop metabolism; others retard or accelerate it. 



Living organisms react to certain drugs in ways that make the proto- 

 plasm unable to get along without these habit-forming drugs. 



The living organism reacts chemically to foreign substances in ways that 

 are generally adaptive. The chemical changes, usually in the blood, result in 

 antibodies that counteract specific poisons or parasites, so that the body 

 becomes temporarily or lastingly immune. 



Serums containing specific anti-substances are used to bring about 

 passive immunity. 



Immunity to certain diseases can be acquired by recovering from them. 



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