Final answer:
Prions are the body substances that do not contain transmissible infectious agents because they lack nucleic acids. They were discovered by Stanley Prusiner, and despite initial skepticism, are now recognized as a unique form of infectious agent.
Step-by-step explanation:
The body substance that is not assumed to contain transmissible infectious agents are prions. Prions are proteinaceous infectious particles that lack nucleic acids such as DNA or RNA. Unlike viruses, bacteria, or other pathogens that contain genetic material and thus can transmit disease through DNA or RNA, prions cause infection through a different mechanism. The infectious nature of prions was established by the groundbreaking work of Stanley Prusiner, who showed that these proteins can cause disease despite their non-nucleic acid composition. The immune system usually targets foreign bodies with a molecular weight more than 5000 kDa; however, prions evade this response as they do not present as typical foreign pathogens due to their protein-only structure.