There is a body of known research from around the mid 20th century that led to the formation of ethical guidelines for research and the establishment of Institutional Review Boards (IRBs). Here is another study in the news that shows why these guidelines are necessary:

The United States apologized on Friday for an experiment conducted in the 1940s in which U.S. government researchers deliberately infected Guatemalan prison inmates, women and mental patients with syphilis.

In the experiment, aimed at testing the then-new drug penicillin, inmates were infected by prostitutes and later treated with the antibiotic.

“The sexually transmitted disease inoculation study conducted from 1946-1948 in Guatemala was clearly unethical,” Secretary of State Hillary Clinton and Health and Human Services Secretary Kathleen Sebelius said in a statement.

A researcher discovered this case while doing research that followed up on the Tuskegee experiments of the 1960s.