A similar ambiguity involves the terms CONTAGION and INFECTION, which are regarded by many authors as synonymous and convertible expressions, signifying the matter or medium by which certain diseases are communicated from one individual to another; while others, on the contrary, confine the term _Contagion_, as its etymology would suggest (_con_ and _tango_) to the communication of those diseases, which can only be transferred by actual contact of the sick, or of the palpable matter from their bodies; and apply the term INFECTION to the communication of those other diseases which spread by means of invisible effluvia. Now we would observe in the first place, that according to the most correct rules of philology, the import of words is not necessarily to be deduced from their derivation, but frequently to be either assumed conventionally according to a definition, or to be adhered to in the sense affixed to it by established usage; in the next place, the distinction which the etymologist would thus establish between the terms _Contagion_ and _Infection_ is not accurate, for in every case of infection, there is an actual contact of morbific matter, whether visible or not, and some diseases, as the _Small-pox_, are communicated both by palpable matter and by imperceptible effluvia.[171] Our best writers[172] have therefore agreed to consider the word _Contagion_ as expressing the morbid poison, or the means of transferring a disease, and _Infection_ as denoting the operation of the poison, or the act of communication of the disease. Dr. _Hancock_[173] very justly observes that in almost all the best Latin writers on medicine, _Contagium_, and _Contagio_ are the only words used to denote the effluvia, or emanations arising in disease, which are capable of infecting the sound, whether _mediately_ by the air, or by infected goods called _Fomites_, or _immediately_ by the touch: to limit contagion therefore to the propagation of disease by contact only, would be to disallow the more comprehensive use of the term in our best authors. Those diseases which occur among the inhabitants of a particular region or place, are said to be ENDEMIC, or ENDEMIAL; thus _Intermittent_, and _Remittent_ fevers, which are occasioned by the miasmata of marshy grounds are _Endemic_ in low countries: the _Goitre_, or bronchocele, connected with that peculiar intellectual imbecility which characterises the CRETIN, is ENDEMIC among the Alps; in these instances, some _local_ cause obviously exists which produces the disease in the respective districts: the disease therefore belongs to the districts, and affects those that reside there, but extends no farther; and hence the distinction between _Endemic_ and _Epidemic_ diseases is obvious and important.