18 GENERAL BACTERIOLOGY 
reciprocal relations with their hosts as parasites or pathogens, relations 
which are neither purely bacterial, animal nor vegetal in their limita- 
tion. A new science, Immunology, is rapidly developing which is 
concerned chiefly with the elucidation of these relationships between 
host and parasite. 
Bacteria are ordinarily classed as plants, but they exhibit several 
prominent (:'haracteristics which suggest a relationship with the lowest 
animals. The most important of these is the absence of photodynamic 
pigment (chlorophyll). This indicates very clearly their dependence 
upon preformed food and therefore implies an analytical or destructive 
function in the economy of Nature. Waksman,i however, has appar- 
ently shown that certain organisms of the genera Thiothrix and Beg- 
giatoa, usually grouped with the bacteria utilize inorganic sulphur and 
sulphur compounds together with simple ammonium salts for energy. 
The great majority of bacteria are saprophytic, living upon dead 
organic matter, which they transform into simple compounds suitable 
for plant use. These bacteria are Nature's analysts. Some are para- 
sitic on living plants and animals; a few are progressively pathogenic 
for man and animals. It is this last group, few in numbers, but for- 
midable in that their activities are in partial opposition to those of 
man and animals, that has given to bacteria all the notoriety which 
they possess. 
The doctrine of contagion, that is the belief that certain diseases were 
transmitted from one person to another, is of great antiquity. The 
Book of Leviticus, in the Bible, and the Talmud contain much of the 
lore of the Jewish priests, who seem to have developed very practical 
methods for the prevention of disease transmission. The early and 
medieval philosophers also seem to have recognized the possibilities of 
infection from one person to another. These speculations culminated 
in the famous De contagionibus et contagionis morbis, by Fracastoro, 
published in L546. His poem "Syphilis sive Morbi Gallici," which 
appeared in 1530 dedicated to his friend Cardinal Pietro Bembo is 
celebrated not only because it contains the word Syphilis, coined by the 
author but also because it gives a contemporary picture of the disease 
by an intelligent observer. 
In the Contagionibus, Fracastoro advances the idea that disease is 
disseminated by small particles ("seminaria," or seeds), and infection 
may arise: (1) By contact only; (2) infection by contact and fomites; 
(3) by contact, by fomites and also transmitted to a distance. He 
draws an analogy between diseases of the first class and putrefactions 
that pass from one body to another; the "seminaria" likewise passing 
from one person to another. The contagion of the second class is much 
like the first, except that the "seminaria" may remain latent in sub- 
stances (fomites) that are porous and more or less "calorific." Con- 
tagion of the third type is less readily grasped. Fracastoro calls atten- 
1 Jour. Bact., 1922, 7, 231. 
