Generally, vulnerable populations refer to groups of people whose range of options is severely limited, who may be subjected to coercion or who may be compromised in their ability to give informed consent to receive medical or surgical treatments or to participate in research. This particular page provides information on vulnerable populations in research.
"There are several definitions available for the term vulnerable population; the words simply imply the disadvantaged sub-segment of the community requiring utmost care, specific ancillary considerations and augmented protections in research...The vulnerable populations refers to but not limited to children, minors, pregnant women, fetuses, human in vitro fertilization, prisoners, employees, military persons and students in hierarchical organizations, terminally ill, comatose, physically and intellectually challenged individuals, institutionalized, elderly individuals, visual or hearing impaired, ethnic minorities, refugees, international research, economically and educationally disabled and healthy volunteers."
Shivayogi P. Vulnerable population and methods for their safeguard. Perspectives in Clinical Research. 2013;4(1):53-57. doi:10.4103/2229-3485.106389
Depending on the search tool that you are using (catalog, database, web search), you may need to search for multiple terms. Here are some terms to consider for both textword and subject searching (remember that some databases have specific subject terms to use).
Combine with population-specific terms:
Also consider searching for historical situations when informed consent was not given:
The immortal life of Henrietta Lacks by 
Community-based Research with Vulnerable Populations : Ethical, Inclusive and Sustainable Frameworks for Knowledge Generation by 
