=Natural Immunity= varies in different animals. Thus enteric fever, scarlet fever, and measles are not known to occur except in man. Tuberculosis, anthrax, hydrophobia (called rabies in the dog), glanders and tetanus are common to man and certain other animals. Man, cattle and pigs frequently suffer from tuberculosis; goats, sheep, horses, and dogs are practically immune to it. =Epidemic and Endemic Diseases.=—Infective diseases may occur _sporadically_, in _epidemics_, or in _pandemics_, _i.e._ epidemics spread over a number of countries. The word =epidemic= is used here to mean specially prevalent, and not to apply only to infective diseases. Thus there may be an epidemic of arsenical poisoning from contaminated beer. Certain infective diseases are =endemic= or topical, _i.e._ they have special homes or centres, from which they occasionally spread as epidemics. Yellow fever, cholera, and malaria belong to this group. In a minor degree enteric fever, epidemic diarrhœa, and tuberculosis may be described as endemic. Each infective disease has a special =seasonal incidence=. Of these the most important are the _autumnal_ group, viz. Epidemic Diarrhœa, maximum prevalence in July and August. Enteric Fever „ „ „ November, but excessive, Aug. to Dec. Erysipelas „ „ „ Nov. to Dec. Diphtheria „ „ „ Nov. and Dec., excessive, Sept. to Dec. Scarlet Fever „ „ „ Oct., excessive in Aug. to Dec. Of other infective diseases Small-pox has its maximum prevalence in May, but is excessive Jan. to June. Whooping Cough „ „ „ Dec. to May. Measles commonly has two seasonal maxima, in June and December with intervening minima. =Causes of Epidemics.=—Measles recurs in the large towns of England every alternate year. Other infective diseases occur at less regular intervals. The recurrence of epidemics is not solely due to personal infection and the accumulation of a population at susceptible ages. There are longer =cycles= of the causes of which but little is known. Thus scarlet fever has been shown by Longstaff and Gresswell to become epidemic chiefly in dry years; and I have shown that diphtheria and rheumatic fever become widely epidemic under the same conditions, diphtheria becoming so only when a series of dry years occur in immediate succession. CHAPTER XLIV. ACUTE INFECTIVE DISEASES. Acute Infectious Diseases are characterised by certain definite characters. 1.—_They are usually infectious or contagious._ It is preferable to use these two terms as interchangeable. The modes in which infection is received vary greatly with different fevers.