588 PROCEEDIXGS OF SECTION I. 



as to the time of onset, water, and milk, and food supply, relation 

 to other cases, length of residence, district, visits to other districts 

 or into zones of infections. (A form suitable for such a purpose is 

 attached). All the work of the investigation should be founded 

 on the known means of spread of the typhoid bacillus, and 

 naturally the more usual and likely possibilities of spread should 

 be first exhausted. Thus water and milk supply should be most 

 carefully considered in relation to the cases, as it is frequently a 

 waste of time to bacteriologically examine either unless a very good 

 prima facie case can be made out epidemiologically. Fly-food or 

 fly-milk carriage are two other important possibilities to be 

 considered. 



It is perhaps most important to remember that typhoid infec- 

 tion is virtually always excretal infection, and that the primary 

 source of the disease is almost invariably the urine or faeces of a 

 previous case. Such interesting possibilities as direct contact, 

 air infection, sputum or sweat infection, and corpse-water 

 pollution, must be regarded by the practical man, in the light of 

 present day knowledge, as possibilities only, and if we are to get 

 the best result for our trouble we would be well advised to confine 

 ourselves, until we can absolutely exclude their action, to the con- 

 sideration of faecal or urinary infection (1) by water, (2) by milk 

 or food, (3) by flies. Faecal or urinary infection is often secondary 

 through contamination of hands. Though sweat may play a part 

 in infection by milk or food, we will not err greatly in considering 

 excreta as the primary source of most outbreaks. 



Having accumulated facts from every case and from the local 

 practitioners, a consideration of these will frequently give one a 

 clue to the origin of the outbreak. 



The great object of the epidemiological investigation is to find 

 the common bond uniting the cases (not necessarily all the cases, 

 for some may be secondarily infected, and some may have an 

 origin quite independent of the general outbreak). We may be 

 able to trace a common water or milk supply in the infected and 

 not in the unaffected; we may be able to find that the cases hav'e 

 been at some dinner (7) or festivity together at the time corre- 

 sponding to infection or in the company of some particular person 

 or in some particular place. Some article of food may have 

 been sold in the district and consumed by the sick persons at that 

 time such as oysters, shellfish. All these facts will be brought 

 out by the case-sheets and card index systems. We may on the 

 other hand be able to trace no such relationship, and tlaen it is 

 for the investigator to find if there is any explanation of the facts 



