It is, therefore, seen that at least three different microbes play an important part in the same disease.
Of late years enormous impulse has been given to our knowledge of the causation of disease by microbes, through the works of Gaspard, who injected putrid matter into the veins of a living animal; by Villemin, who discovered that tuberculosis is infective; by Davaine; and especially by Pasteur, Koch and others too numerous to mention, who have worked, and are still working, at the microbic causation of disease with marvellous success.
The greater the number of leucocytes that can reach the spot where the invading microbes enter the more quickly can the microbes be destroyed and general infection prevented.
The microbes appear in many cases to attract the leucocytes (positive chemiotaxis), but when very virulent they usually repel the leucocytes (negative chemiotaxis) and excrete toxins which kill the leucocytes.
The irritation caused by the microbes generally is followed by dilatation of the vessels of that part and thus more leucocytes are brought up to the fight.