12 INFECTION AND IMMUNITY 



the microscope grouped in pairs, or in chains, or in 

 irregular masses. 



Some of the bacteria multiply not only by binary 

 division but also by the formation of spores, which 

 correspond, so far as the preservation of the species 

 is concerned, to the seeds of higher plants. The 

 growing cells are delicate plants which are easily 

 killed by heat and by various chemical agents (dis- 

 infectants). But the spores resist a much stronger 

 solution of germicidal agents and a much higher 

 temperature. They also resist desiccation, and may 

 retain their vitality for months or years until circum- 

 stances are favourable for their development, when, 

 under the influence of heat and moisture, they repro- 

 duce the minute microscopic plant bacillus or spiril- 

 lum and multiplication by binary division again 

 occurs. It is fortunate that comparatively few patho- 

 genic bacteria produce spores, for if this were the 

 case it would be a much more difficult task to arrest 

 the progress of an epidemic of such diseases as 

 typhoid fever, bubonic plague, cholera, or diphtheria. 

 The only infectious disease of man in which spores 

 have been demonstrated to be formed is tetanus, or 

 lockjaw. As this disease is not likely to be com- 

 municated by the sick to those associated with them, 

 either directly or indirectly, the formation of spores 

 by the tetanus bacillus is not so serious a matter. 



