In the common moulds there are three genera which are frequently represented, namely, Penicillium, Aspergillus, and Mucor, and of these the aspergillus is the only one which is of importance as an occasional parasite.

Aspergillus glaucus is one of the common greyish-blue moulds. According to De Bary, it is not a true aspergillus, but belongs to the genus Eurotium. It has been described as occurring as a parasite, but this has probably been a mistake for one of the true aspergilli. It does not grow at a temperature approaching that of the body.

Aspergillus fumigatus forms a greenish mould with a granular surface, becoming grey later on. It grows best at a high temperature, and its spores may, after introduction into the body of animals, grow there (see below).

Aspergillus flavus is a yellow or yellowish-green mould, while Aspergillus niger has a dark brown colour. These also grow at the temperature of the body, and may become parasitic.

The Aspergilli have been introduced by experiment Into the bodies of animals, and the spores of some of them, but especially A. fumigatus and flavus, are able to live and germinate there. They do not, however, like the bacteria, multiply in the body, but they may by their growth produce destructive effects. The spores carried by the blood settle in certain organs, and may produce lesions not unlike metastatic abscesses. In a paper by Grawilz, for example, there is an illustration showing lesions in the kidney closely resembling those occurring in pyaemia. In each of these lesions a growing fungus is found (see Fig. 159), which is causing necrosis and inflammation of the tissue, but is not producing spores.

Fig. 159. - Fungus growth in kidney. To the left of the figure the kidney tissue has been cleared up by adding solution of soda so as to bring out the fungus (aspergillus). Fat drops and crystals of tyrosin are present, but no spores. (Grawitz).

Grawitz believed that ordinary mould fungi may, by cultivation, be so altered as to possess pathogenic properties such as those just mentioned, but subsequent observation shows that it is only the aspergilli which have these powers, and that they have them without any special cultivation.

In man aspergillus has been found not infrequently in the lung, usually in the walls of pulmonary cavities. Such cases are described under the name of Pneumonomycosis. The general extension of aspergillus or other fungus has been observed.

Aspergilli are occasionally found growing in the ear, constituting an Otomycosis. They occur by preference on the tympanic membrane and inner third of the external meatus, but when the drum is perforated they may extend to the middle ear. The fungus does not apparently grow unless there be some previous disease causing a breach in the epithelium, and they do not penetrate deeply into the structures. The forms observed have been chiefly aspergillus fumigatus, flavus and niger, but other kinds have been found.

Leber has also observed a fungus growing on the cornea of man. It was an aspergillus, and he determined by experiment that it was capable of growing on the rabbit's cornea.

A mycosis of the Nasal mucous membrane has also been observed; the fungus was aspergillus fumigatus.