INCUBATION PERIOD 273 



fever or diphtheria this incubation is a short one of three to five days. 

 Now, whilst there is no hard-and-fast Hne strictly delimiting the 

 period of incubation for any of the infectious diseases, it has been 

 observed that, when such diseases are conveyed by milk, the incuba- 

 tion period is shorter than the normal. There are not a few 

 epidemics which show that scarlet fever and diphtheria may com- 

 mence to reveal their presence in a few hours, and typhoid fever in 

 a few days. Yet this varies, and no two epidemics are precisely 

 the same. In the Clifton typhoid epidemic of 1897, to which 

 reference has been made, there were two cases of girls who had 

 each only consumed the implicated milk on one occasion, and hence 

 the incubation period was ascertained with fair precision. In both 

 cases it was under seven days. At Great Harwood in Lancashire 

 in 1895 the incubation period in typhoid was, in some cases, as 

 short as two days (Sarjeant). In Marylebone in 1873 a girl is 

 said to have drunk two pints of the infected milk once, and 

 sickened within five days. In contrast to this was a case observed 

 by Power in which the attack of typhoid fever followed a single 

 glass of milk after an interval of three weeks. 



Scarlet fever carried by milk may have almost no incubation 

 period at all In the Fallowfield epidemic, near Manchester, in 

 1899, Dr Airy found that in some cases attacked the incubation 

 period was less than twenty-four hours. In a recent London 

 epidemic of scarlet fever, one of the writers traced several cases 

 where the incubation period was under twenty-four hours. 



Where the incubation period is abnormally short, the symptoms 

 which follow resemble an " intoxication " rather than an infection. 

 In this particular, milk outbreaks follow the behaviour of food 

 poisoning outbreaks. We may digress for a moment to note the 

 chain of events. An individual in perfect health partakes of some 

 poisonous food, and shows signs of illness immediately thereafter. 

 There is no incubation period. On investigation the chemist suc- 

 ceeds in separating, from the poisonous article of food, an alkaloidal 

 poison, a ptomaine, i.e. d. product of organismal action. The condi- 

 tion has been one of intoxication. Another individual, in perfect 

 health, also partakes of some poisonous food, and shows signs of 

 illness some hours or even some days thereafter. There is an in- 

 cubation period, during which the infective agent is forming its 

 poisonous products in the blood and tissues of the patient. On 

 investigation, if within a sufficiently short period of time, the bac- 

 teriologist isolates from the poisonous article of food an organisviy 

 a vegetable cell, acting as an infective agent This was present, 



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