Hippocrates was the first physician to use the term "miasm". He thought that certain infectious diseases were transmitted to humans by air and water tainted by miasms.
In late 18th century it was a common belief that miasms were impure airs that were responsible for the spread of epidemic diseases among groups of people. Hahnemann (homeopathy) realized that the air could carry infectious diseases but he did not consider the pathogenic material to be gaseous in nature. In homoeopathic this idea is still important, but in a bit different way.