The affections in this group show in their structure and general relations considerable analogies to inflammation on the one hand and tumours proper on the other. Hence the conditions are variously called Specific Inflammations, Specific New-formations or Infective Tumours. As already indicated, the expression 'infective' means that the disease is ' spreading,' that it depends on some virus which propa gates itself, and tends to reproduce the same kind of lesion outside its original seat. In most of the forms of disease included here, the exact nature of the virus has been made out, and in all of these except one it is found to be a microbe in the form of a short rod-shaped bacillus. It may perhaps be legitimately inferred that in the rest specific microbes are the infective agents. Several of these diseases are not only infective but infectious, communicable, that is to say, from person to person; some are capable of being inoculated into animals.

The virus has, to begin with, a local seat, where it acts in a concentrated form, inducing new-formation of tissue and inflammatory phenomena. The new-formation has many features in common with that of inflammation, but it has also certain distinctive or specific features. It consists chiefly of structures analogous to granulation tissue, not simply, as in inflammations, replacing a part of the normal tissue, but forming more or less independent masses which resemble tumours. Hence, the group of diseases included here is often designated Granulation-tissue tumours or Granulomata. Outside these more specific new-formations, there are usually the ordinary lesions of inflammation, the virus having acted in a less concentrated form.

But the tissue of the tumours presents certain differences from ordinary granulation tissue, chiefly in its tendencies. The granulation cells tend to undergo fatty degeneration and necrosis, and so the tissue may become caseous or break down. At the same time there is the more normal tendency to undergo development into connective tissue, and this may go on in an imperfect way alongside the other change. Hence, the tumours frequently present great varieties in structure, and it is sometimes difficult fully to unravel their relations.