In homeopathy a miasm is a disease entity that can enter the body similar to an infectious process or they can be present at birth and lie dormant until something triggers its activity.   The trigger is usually attributed to unhealthy lifestyle, trauma, emotion, etc.  The miasm will become apparent first as a symptom and then as a disease.

In-depth explanation of miasms from Homeopathy Online

Click here to view a homeopathy miasm chart

In conventional medicine, a miasm is a term that refers to noxious effluvia or emanations that were once thought to carry malaria and other epidemic diseases.  This notion fell into disuse with the discovery of the microbial causes of these diseases.