Disease can be defined as a change in the body processes that impairs its normal ability to function. Every day the physiology of the human body demands that oxygenation, acidity, salinity, and other functions be maintained within a very narrow spectrum. A deviation from the norm can be brought about by organ failure, toxins, heredity, radiation, or invading bacteria and viruses.
Normally the body has the ability to fight off or to neutralize many pathogenic organisms that may gain entrance through an opening in the skin or by other means. The immune system mobilizes quickly to rid the body of the offending alien and restore or preserve the necessary internal environment. Sometimes, however, the invasion is one that is beyond body's resistance, and the immune system is unable to overcome the invader. A disease may then develop. When the internal functions of the body are affected to the point that the individual can no longer maintain the required normal parameters, symptoms of disease will appear.
The infection brought about by a bacterium or virus usually generates specific symptoms, that is, a series of changes in the body that are characteristic of that invading organism. Such changes may include development of a fever (an internal body temperature higher than the norm), nausea, headache, copious sweating, and other readily discernable signs.
Much more important to the physician, though, are the internal, unseen changes that may be wrought by such an invasion. These abnormalities may appear only as changes from the norm in certain chemical elements of the blood or urine. That is the reason patients are asked to contribute specimens for analysis when they are ill, especially when their symptoms are not specific to a given disease. The function of organs such as the liver, kidneys, thyroid gland, pancreas, and others can be determined by the levels of various elements in the blood chemistry.
For a disease that is considered the result of a pathogenic invasion, the physician carries out a bacterial culture. Certain secretions such as saliva or mucus are collected and placed on a thin plate of culture material. The bacteria that grow there over the next day or so are then analyzed to determine which species are present and thus, which antibiotic would be most effective in eradicating them.
Viruses present special challenges, since they cannot be seen under a microscope and are difficult to grow in cultures. Also, viruses readily adapt to changes in their environment and become resistant to efforts to treat the disease they cause. Some viral diseases are caused by any number of forms of the same virus. The common cold, for example, can be caused by any one of some 200 viruses. For that reason it is not expected that any vaccine will be developed against the cold virus. A vaccine effective against one or two of the viruses will be completely useless against the other 198 or 199 forms.
The agents that cause a disease, the virus or bacterium, are called the etiologic agents of the disease. The etiologic agent for strep throat, for example, is a bacterium within the Streptococcus genus. Similarly, the tubercle bacillus is the etiologic agent of tuberculosis.
Modern medicine has the means to prevent many diseases that plagued civilization in the recent past. Polio, a crippling disease brought about by the poliomyelitis virus, was neither preventable nor curable until the middle 1950s. Early in that decade an outbreak of polio affected an abnormally large number of young people. Research into the cause and prevention of polio immediately gained high priority, and by the middle of the decade Dr. Jonas Salk had developed a vaccine to prevent polio. Currently all young children in developed countries can be vaccinated against the disease.
Similar vaccines have been developed over the years to combat other diseases that previously were lethal. Whooping cough, tetanus, diphtheria, and other diseases that at one time meant certain death to victims, can be prevented. The plague, once a dreaded killer of thousands, no longer exists among the human population. An effective vaccine has eradicated it as a dread disease.
The resistance to disease is called immunity. A few people are naturally immune to some diseases, but most have need of vaccines. This type of immunity, attained by means of a vaccine, is called artificial immunity. Vaccines are made from dead bacteria and are injected into the body. The vaccine causes the formation of antibodies, which alert the immune system in the event a live bacterium invades.
The body's immune system, responsible for guarding against invading pathogens, may itself be the cause of disease. Conditions such as rheumatoid arthritis and Lupus are considered to be the result of the immune system mistaking its own body for foreign tissue and organizing a reaction to it. This kind of disease is called an autoimmune disease—auto, meaning one's own, and immune referring to the immune system. Scientists have found that little can be done to combat this form of disease. The symptoms can be treated to ease the patient's discomfort or preserve his life, but the autoimmune reaction seldom can be shut down.