Not only are some infectious diseases transmissible to others, a unique characteristic among human diseases, but their transmission mechanisms are relatively few (including inoculation and airborne and waterborne transmission), well understood, and comparatively easy to study, both experimentally and in the field. In addition, such transmission is generally amenable to medical and public health interventions. Unlike many chronic and lifestyle-associated diseases resulting from multiple, interacting risk cofactors, most infectious diseases are caused by a single agent, the
identification of which typically points the way not only to general disease-control measures (e.g., sanitation, chemical disinfection, hand washing, or vector control) but also to specific medical measures (e.g., vaccination or antimicrobial treatment).
Please be aware that I'm neither a native English speaker nor a teacher.
That would be my understanding, with "in the field" referring to actual experience of the transmission among humans.