Homeopaths have known for 180 years that there can be predispositions to illness, and that these can be passed on. Dr Samuel Hahnemann (1755-1843), the discoverer of the principles of homeopathy, believed that living organisms were responsible for disease. He used the term ‘miasm’ for these organisms, a term generally used at that time for bad air which caused illness:
On board ships – in those confined spaces, filled with mouldy watery vapours, the cholera-miasm finds a favourable element for its multiplication, and grows into an enormously increased brood of those excessively minute, invisible, living creatures, so inimical to human life, of which the contagious matter of the cholera most probably consists – on board these ships, I say, this concentrated aggravated miasm kills several of the crew ... 
It was another 60 years before Koch saw these organisms, and later on orthodox medicine gave them the name ‘germs’ (from the word for seeds).
After extensive research Hahnemann identified three diseases which, if the symptoms were suppressed, he claimed led to other long-lasting (chronic) conditions and predispositions to illness. These three diseases he called:
- Psora (from the Greek for itch) – the ‘itch’ disease from scabies;
- Syphilis – syphilis;
- Sycosis (from the Greek for fig since this disease produces fig-like warts) – gonorrhoea.
Nowadays homeopaths use the term ‘miasms’ for the predispositions and tendencies to illness after diseases have been suppressed. These predispositions and tendencies have their own individual patterns of symptoms, and remedies can be selected to take this larger picture into account and so treat the underlying process of the individual’s condition.
Hahnemann maintained that the oldest of these miasmatic diseases was psora, and he noted that
The fact that this extremely ancient infecting agent has gradually passed, in some hundreds of generations, through many millions of human organisms and has thus attained an incredible development, renders it in some measure conceivable how it can now display such innumerable morbid forms in the great family of mankind. 
This was published 31 years before Darwin proposed his theory of evolution in On the Origin of Species, so Hahnemann's assertion of the "development" (i.e. evolution) of the itch disease represents again his extraordinarily modern views.
Hahnemann’s theory also tends to be confirmed by the fact that homeopaths at the beginning of the twentieth century claimed to have discovered a fourth miasmatic disease:
- Tuberculosis – tuberculosis (also known historically as phthisis).
In fact it was only towards the end of the nineteenth century that it became possible to suppress the symptoms of this very ancient disease, and therefore create the miasmatic condition. 
An important aspect of miasmatic conditions is that they do not get better without appropriate treatment, but get steadily worse throughout a person’s life. The actual symptoms may change (within the limits of each miasm), and they come under the headings of a number of chronic conditions, but the degree to which they affect the body will become more severe. Within this framework there may also be acute episodes which can be fatal: asthmatic attacks, heart attacks, strokes, allergic responses, etc. Homeopaths distinguish between emergency treatment for acute episodes, when an orthodox medical intervention may be life-saving, and treatment of the underlying miasmatic condition which predisposes to such episodes.
1. Samuel Hahnemann, Appeal to Thinking Philanthropists Respecting the Mode of Propagation of the Asiatic Cholera, (Leipzig: the author, 1831), translated by R E Dudgeon, M.D. in The Lesser Writings of Samuel Hahnemann, 1851 edition (New Delhi: B Jain Publishers, repr. edn 2002), p. 758.
2. Samuel Hahnemann, Organon of Medicine, manuscript completed 1841, 1st English edn 1921 (Calcutta: Roy Publishing House, repr. edn 1972), § 181 p. 160.
3. William Alderson, ‘Miasms and Social Change’, Homoeopathic Links, 21 (2008) 117-122 p. 121.