406 PLANTS AND MAN 



attacking organisms themselves or against the poisons, or toxins, 

 produced by the bacteria. Thus when an individual has recovered 

 from an attack of smallpox, measles, or scarlet fever, his body 

 has produced such a strong immunity reaction to the micro- 

 organisms or their toxins that subsequent attacks usually do not 

 result in a recurrence of the disease. But today it is not necessary 

 to have had a disease to be insured against further attack; arti- 

 ficially acquired immunity, which is just as effective as naturally 

 acquired immunity, may be had without the risk and discomfort 

 of having the disease. Such an immunity is brought about in 

 several ways, one consisting of injecting into the human body 

 cultures of the causal organism which have previously been 

 weakened, or reduced in virulence by passing them through the 

 body of another animal. This is done in the case of the organism 

 causing smallpox. Calves are inoculated with the virus — sup- 

 posedly a microorganism too small to be seen or isolated — which 

 is collected in a weakened condition from pustules on the skin 

 and manufactured into smallpox vaccine for human use. This 

 stimulates an immunity reaction just as does the presence of the 

 disease organism in its normal form, but without the ill effects 

 of the latter. 



Another method, used in creating immunity to typhoid fever 

 and Asiatic cholera, is to inoculate with dead bacteria in known 

 numbers, thus stimulating an increasing immunity reaction with 

 increasingly larger doses of the killed bacteria. A third method 

 of creating artificial immunity is by the use of bacterial toxins, 

 or the poisons given off by bacterial cells. This is used in the 

 case of immunity to diphtheria, where the bacterial cells are 

 grown for several days in a meat, broth, then killed and the toxins 

 filtered free of any bacterial cells and prepared for injection in 

 a chemically modified form known as a toxoid. This has replaced 

 the older method of inoculating a horse with the bacteria and 

 using the serum from horse blood as an antitoxin to be adminis- 

 tered with the toxin. 



Often an individual has been exposed to a disease or is suffer- 

 ing from the early stages of the disease before preventive measures 

 are taken. This is called passive immunity since instead of inject- 

 ing an immunity stimulating substance into the body, the im- 



