In the late 1970s and 1980s, as the AIDS (Acquired Immune Deficiency Syndrome) pandemic was sickening and killing millions of people around the world, scientists worked relentlessly to identify the causative agent of the disease in the hopes of developing effective drugs and vaccines against it. In 1983, HIV (Human Immunodeficiency Virus) was identified as the cause.
How do scientists determine what causes an infectious disease and how do they prove it? The first conclusive demonstration that bacteria could cause disease was described in the work of Louis Pasteur and Robert Koch. Working independently, each scientist demonstrated that anthrax, a serious disease in domestic animals, which is also transmissible to humans, was caused by bacteria found in the bloodstream. The work of Pasteur, Koch, and other scientists in the field ushered in an era of discovery in which bacteria, viruses, and parasites were shown to be the causes of infectious diseases around the world.
In 1876, Koch proposed a set of criteria by which a microorganism could be determined to be an infectious agent (or pathogen). These criteria included:
- The microorganism must be present when the disease is present but absent in healthy organisms.
- It must be possible to isolate the microorganism.
- The isolated microorganism must cause disease when placed into a healthy organism.
- It must be possible to re-isolate the microorganism from the second diseased host.
These criteria, called Koch’s Postulates, are still used today to determine whether a disease is caused by an infectious agent. All organisms live in some kind of environment that provides them with nutrients and shelter in which to grow and replicate. In some cases, an infectious agent can survive in a number of different environments such as soil, water, a plant, or an animal, and it is only a matter of chance where the agent finally appears. In other cases, as you have seen in your reading, an infectious agent, for one reason or another, can live only within another organism—the host. Often the reason for this is that these pathogens require the nutrients that the host organism provides. The host becomes the environment from which the infectious agent derives everything it needs to survive. These organisms can only survive in one specific environment, which may be a plant or an animal or a bacterium. In some cases, the specificity may even extend to the type of tissue or cell in which the pathogen must live.
Wherever it lives, an infectious agent must grow and replicate. To do this, it must locate a suitable environment, take up nutrients, and release byproducts of its own metabolism. In carrying out these processes of life, the organism may deplete the nutrients in the environment, release toxic substances, and cause physical damage to its surroundings. The depletion of nutrients, damage from toxic substances, and mechanical damage can all contribute to causing the symptoms characteristic of the host’s disease associated with that infectious agent.
Scientists working to prove that HIV was the cause of AIDS were able to fulfill the first two criteria of Koch’s Postulates. For obvious ethical reasons, they could not fulfill the last two. However, the availability of animal models that demonstrated the same disease caused by a similar virus allowed researchers to demonstrate that HIV was responsible for this devastating disease by fulfilling all four of Koch’s Postulates.