infection

Although the "germ theory of disease" is most often considered to be a relatively modern concept - and direct identification of infectious microorganisms awaited Robert Koch's 1875 discovery of the Antrax bacillus - we can find reference to the notion of contagium vivum (that miniscule/invisible self-procreating living entities are responsible for some communicable diseases) in the Atharvaveda (late 2nd millennium BC), with Western references as early as 36 BC in Marcus Terentius Varro's Agricultural Topics in Three Books.

Hahnemann demonstrated an amazing appreciation of the role that microscopic living organisms might play in disease provocation - e.g., his 1831 description of the mode of propogation of the Asiatic Cholera, published 3 years after the publication of his Chronic Diseases:

the cholera-miasm finds a favourable element for its multiplication, and grows into an enormously increased brood of those excessively minute, invisible, living creatures ... The cause of this is undoubtedly the invisible cloud that hovers closely around the sailors ... which is composed of probably millions of those miasmatic, animated beings ... this pestiferous, infectious matter ... the invisible (probably animated) and perpetually reproductive contagious matter ... (Samuel Hahemann, The Mode of Propogation of the Asiatic Cholera; Leipzig, 1831; included in the collected Lesser Writings of Hahnemann)

The full text of this pamphlet is really quite remarkable, deserving a very central place in the history of the germ theory of disease - find it here.

Hahnemann used the term "infection" 60 times in the Principles section of the Chronic Diseases, referencing the means of acquiring chronic disease.  In the Organon, he stated:

Chronic diseases arise from dynamic infection by a chronic miasm. (Hahnemann, Organon, aph.72)

We've identified today the Spirochete responsible for Syphilis (Treponema pallidum, identified by Fritz Schaudinn in 1905), and the virus responsible for Sycosis (human papillimavirus; papilloma viruses were first identified in 1933) - and today we clearly appreciate the infectious nature of these diseases.

At the time of his writing of The Chronic Diseases, Hahnemann envisioned these first two chronic diseases - Syphilis and Sycosis - as special cases of contagia vivum; infection by a dynamic, living organism, aquired during a person's lifetime; unique in that the dynamis was seemingly not capable of resolving these without external assistance.