But the major health concerns today are much more complicated than these infectious diseases with a single microorganism as the cause. Today we are dealing with diseases such as cancer, heart disease, and autoimmune disease in which the etiology is much more complex. We cannot always predict who is susceptible to these diseases and to what degree. And when we go back and look at infectious diseases, a great deal of variation in susceptibility is seen here as well. If a group of people, not previously exposed to an organism, receives the same amount of exposure to the same pathogen, one may become fatally ill while another individual experiences only mild symptoms.
Every human being has the same immune system with the exception of genetic or mutational variation.
Obviously another factor is involved in health maintainence of the individual. Today evidence supporting this idea is overwhelming. What is not clear is how this happens. And because it is impossible to predict or control these influences, it has been difficult to address them from a scientific perspective. But finally scientists are beginning to look at this problem.