In order to find a cure for infectious disease, the infectious disease must be studied. In research labs today, scientists use cell culture (different types of cells modified to grow in plastic plates) to study how pathogens affect cells. In order to know how it affects a whole body system, they must study the infectious disease in a real infection of a person or an animal.
The ethics of using research animals is a debate for another day. Various animals are currently used in medical research, and safety protocols are very closely followed. After a long meticulous road, human subjects can be used when all of the risks have been minimized through previous tests.
However, this was not always the case. There have been some real life "mad scientists" who have used unwilling human subjects to study infectious diseases. Did these studies provide valuable information? Yes. Were they deemed unethical? Absolutely.
A famous case was the Tuskegee Syphilis Study. Between 1932 and 1972 a study was conducted by the United States Public Health Service to study the effects of syphilis. At the start of the study there was no effective drug against syphilis, and the study enrolled 600 black men, both with syphilis and without syphilis. They were observed and monitored in what began as a proper research study.
However, when penicillin was discovered in the 1940's as an effective treatment for syphilis the patients were neither told that they had syphilis nor were they treated for it. This led many of them to eventually die of syphilis or complications of it. They also unknowingly passed the disease on to their wives and partners, and their children were born with congenital syphilis. The unethical behavior of the physicians and researchers in this trial led to further federal laws and regulations to protect human subjects in medical studies.
Although it is hard to know exactly what happened 500 years ago, most anthropologists agree that a large portion of the American indigenous population was decimated by infectious disease. These diseases were introduced to them by the Europeans arriving on their shores carrying European pathogens with them.
The human immune system works better against things that it has already seen before. Remember, a person develops an army of antibodies and then leaves a small militia behind in case the disease returns. The indigenous people had no army and no militia; therefore, when they became infected with new pathogens they had little defense.
Smallpox was the most deadly infectious disease, but it was likely a combination of several new pathogens at once that killed so many people. In addition since so many people were infected for the first time at the same time, not many healthy people were left were able to serve as caregivers, which likely raised the death toll. Some estimates say that millions of indigenous people, possibly 80-90% of the population, were wiped out by diseases introduced by the Europeans.