Still another list might be given of diseases whose general nature indicates that they are caused by bacteria, but in connection with which no distinct bacterium has yet been found.  As might be expected also, a larger list of animal diseases has been demonstrated to be caused by these organisms.  In addition, quite a number of species of bacteria have been found in such material as faeces, putrefying blood, etc., which have been shown by experiment to be capable of producing diseases in animals, but in regard to which we have no evidence that they ever do produce actual disease under any normal conditions.  These may contribute, perhaps, to the troubles arising from poisonous foods, but can not be regarded as disease germs proper.

Variability of pathogenic powers.

As has already been stated, our ideas of the relation of bacteria to disease have undergone quite a change since they were first formulated, and we recognise other factors influencing disease besides the actual presence of the bacterium.  These we may briefly consider under two heads, viz., variation in the bacterium, and variation in the susceptibility of the individual.  The first will require only a brief consideration.

That the same species of pathogenic bacteria at different times varies in its powers to produce disease has long been known.  Various conditions are known to affect thus the virulence of bacteria.  The bacillus which is supposed to give rise to pneumonia loses its power to produce the disease after having been cultivated for a short time in ordinary culture media in the laboratory.  This is easily understood upon the suggestion that it is a parasitic bacillus and does not thrive except under parasitic conditions.  Its pathogenic powers can sometimes be restored by passing it again through some susceptible animal.  One of the most violent pathogenic bacteria is that which produces anthrax, but this loses its pathogenic powers if it is cultivated for a considerable period at a high temperature.  The micrococcus which causes fowl cholera loses its power if it be cultivated in common culture media, care being taken to allow several days to elapse between the successive inoculations into new culture flasks.  Most pathogenic bacteria can in some way be so treated as to suffer a diminution or complete loss of their powers of producing a fatal disease.  On the other hand, other conditions will cause an increase in the virulence of a pathogenic germ.  The virus which produces hydrophobia is increased in violence if it is inoculated into a rabbit and subsequently taken from the rabbit for further inoculation.  The fowl cholera micrococcus, which has been weakened as just mentioned, may be restored to its original violence by inoculating it into a small bird, like a sparrow, and inoculating a second bird from this.  A few such inoculations will make it as active as ever.  These variations doubtless