What is the difference between a primary pathogen and a secondary (opportunistic) pathogen?
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A primary pathogen is a virus, bacteria, fungi, or any other biological entity that causes a disease when it gains entry into a victim's body. The disease caused by the primary pathogen is usually one that stresses the victim's immune system. The immune system is preoccupied in either fighting the microbes that have caused the primary disease or in many cases weakened to a large extent by the disease and the treatment that follows.
This gives an opportunity for other micro-organisms that either live in the body without causing any ill-effects as their number is kept under control by a healthy immune system or those that are present in the external environment but which can be easily dealt by a healthy immune system to grow in number and start to cause problems of their own. Secondary or opportunistic pathogens refers to this group of micro-organisms.
Examples of primary pathogens are HIV and micro-organisms that cause measles, malaria, etc. Secondary pathogens include bacteria and fungi that live in the gastrointestinal tract and are harmless under normal conditions but which can cause serious problems in a person affected by the the diseases mentioned earlier.
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