The origins of medicine began with prehistoric people who believed that diseases were derived from supernatural powers. To destroy the evil spirits, they performed trephining, which involved cutting a hole in the victim's skull to release the spirit. Skulls have been found in which the trephine hole has healed, demonstrating that people did survive the ritual, although it may be assumed that trephining did little to help the afflicted person. The first doctors, known as medicine men, also used herbal concoctions, ritual dances, and incantations to heal their patients.

In about 3000 B.C., the ancient Egyptians developed a systematic method of treating illnesses, which introduced the notion of specialization within the field of medicine. The famous physician Imhotep was so respected that the Egyptians regarded him as the god of healing.

The Greek physician Hippocrates was the first person to declare that disease was caused by natural, not supernatural, phenomena. He introduced a method of conduct and ethics for the practice of medicine. To this day, each physician pledges the Hippocratic Oath on their day of graduation from medical school.

Another Greek physician, Galen of Pergamum, studied in Rome during the second century A.D. He worked as physician to Emperor Marcus Aurelius and lectured to physicians on dissections and experimental physiology. He conducted anatomical studies of animals, particularly apes, because the dissection of humans was illegal. He is credited with the discoveries of blood transport by arteries, the pumping mechanism of the heart, and the function of the kidneys. His written work remained influential for hundreds of years, and during the 15th and 16th centuries, many physicians repeated his experiments to gain further insight into the mechanism of human anatomy.

Avicenna, from Persia, was another major contributor to the early development of Western medicine. His single largest contribution was the book The Canon of Medicine, which included the information of Greek and Arabic physicians that had been gathered from many generations, as well as some of his own findings. The book remained the most important publication for medicine through the 16th century and served as a major resource of information for Eastern and Western countries.

When dissection of human corpses was accepted in the 1500s, Andreas Vesalius was able to conduct his own examinations and correct many of the errors that Galen had made. Vesalius published On the Structure of the Human Body in 1543, which provided the basis for future study of human anatomy.

William Harvey made an important contribution to medicine in the 17th century. Using the observations that Hieronymus Fabricius, a surgeon and pioneering embryologist, had made on the valves in veins, Harvey conducted physical tests to prove that blood circulates through the body and through the veins and arteries. He wrote On the Motion of the Heart and Blood in Animals in 1628.

It was the development of the microscope that moved medical study into the next plane of understanding. Zacharias Janssen, a Dutch eyeglass maker, discovered the benefits of combining magnifying lenses. He is credited with developing the first compound-lens microscope around 1590.

Another Dutch scientist, Anton Van Leeuwenhoek, used microscopes to study the microscopic contents of water, blood, and other body fluids and tissues. He described bacteria from his observations, becoming the first person to recognize the presence of foreign bodies in human fluids.

Discoveries were made more rapidly once the concept of germ infection became recognized and accepted. The awareness of bacteria, fungi, and viruses led to concepts, taken for granted today, that proved to be a major boon to the medical profession. Washing hands before surgery, examinations, and deliveries of newborns led to a decrease in cases of infections and death. Joseph Lister developed the concept of an antiseptic environment that promoted sterilized equipment and surroundings in medical work.

Louis Pasteur successfully produced vaccinations that battled diseases. In the mid-1800s, Pasteur inoculated sheep against a common animal disease called anthrax. He went on to develop a vaccination against rabies, demonstrating that vaccinations were as successful in preventing disease in humans as they were in animals.

Another discovery that the modern world relies on every day is the development of anesthesia. Surgery had been performed without it for hundreds of years, but it was hazardous and extraordinarily painful. In 1846, a surgery by William Morton in Boston before a group of physicians proved to the medical world that the use of anesthesia freed the patient from surgical pain and allowed the physician to work more accurately, more thoroughly, and more extensively than ever before. Early anesthetics included nitrous oxide, ether, and chloroform.

Drugs were discovered that battled and killed some bacteria and infections. Penicillin was discovered by Alexander Fleming in 1928. Howard Florey and Ernst Chain were able to isolate the penicillin compound in pure enough quantities to use it to combat infections such as staph. That advance in 1938 led to mass production, which allowed the British and American armies to use it on wounded soldiers through World War II. The number of lives saved by penicillin is beyond calculation.

Immunologists and bacteriologists experimented with methods of inoculating soldiers against viral infections. Tetanus antitoxin was developed by Emil von Behring and Shibasaburo Kitasato in 1890. The two also worked on diphtheria antitoxin, which, only after the vaccinations had a combination of toxins and antitoxins, would produce an immune response in humans.

During the 1930s, technology was developed that allowed immunologists to isolate and cultivate viruses for study and experimentation. As viruses were isolated, the proper vaccinations could be developed that would trigger immunization in humans. The most heralded of the vaccines was the one developed by Jonas Salk for poliomyelitis (polio). Albert Sabin developed an oral vaccine soon after, which also immunized against polio. Polio had infected hundreds of thousands of people in the United States between 1940 and 1959 and had killed 26,635 people, according to the National Center for Health Statistics. Initial use of the vaccination began in 1954. In 1960, thanks to the vaccination, only 3,190 Americans developed the disease.

Viruses still plague the population and therefore impact on the medical community. Acquired Immunodeficiency Syndrome (AIDS) is the most deadly virus to affect the population. By 2007, 583,298 people had died from AIDS in the United States, according to the Centers for Disease Control (CDC), and the disease had been diagnosed in 468,578 others. Severe acute respiratory syndrome (commonly known as SARS) is a viral respiratory illness that was first reported in Asia in 2003. Before the outbreak was contained, SARS spread to more than 24 countries in North America, South America, Europe, and Asia. According to the World Health Organization, 8,098 people worldwide contracted SARS during this outbreak and 774 of that group died. Researchers are attempting to find a preventive vaccination and a cure for these deadly viruses.

Today doctors also face challenges fighting bacterial diseases. Hailed as miracle drugs in the 20th century, antibiotics have lost some of their effectiveness due to the rise of antibiotic-resistant bacterial strains, such as methicillin-resistant Staphylococcus aureus (MRSA) and Mycobacterium tuberculosis (multidrug resistant TB). Doctors are seeking new treatments and strategies to combat these bugs.

The health care industry continues to develop at a rapid rate with the discoveries of new drugs, treatments, and cures. Modern technologies, such as computers and virtual reality, are being used by the medical community to perform tests, compile data, diagnose illnesses, and train professionals. Many surgeries are no longer performed with a scalpel, but with lasers. Disease, illness, and injury are now being treated and cured so successfully that the general population is living much longer and the number of elderly is increasing.

Scientists are conducting promising research in the field of genetics. Knowledge about genes could help doctors identify people who have genetic predispositions to certain diseases and perhaps lead to a cure for such illnesses as Parkinson's disease and some types of cancer.

The medical and health care field has become one of the largest and most varied occupational areas. Health care workers are employed as physicians, nurses, nursing aides, technicians, technologists, therapists, and in a host of other occupations.