There are about 1 trillion microbial species which constitutes about only 0.0001% of the entire species. One trillion itself is a huge number and they constitute the number of identified species only. Most of the microbes are packed inside the glaciers and is said that global warming which melts the glaciers can release them! The microbes can be broadly classified into 6:-

  • Bacteria
  • Archaea
  • Fungi
  • Protozoa
  • Virus
  • Algae
some shapes and forms of the microbes

An important thing is that even though there are these many species of microbes, we do not get infected by them so often. To understand this it is trivial to know about the infection. Infection is defined as the invasion of the host’s body tissues by disease-causing microorganisms followed by their multiplication, colonization, reaction with the host, and release of toxic product metabolites. Infection is a complex process in which the harmful organisms enter into the host, finds a suitable and weak target place, establishes themselves well, multiples themselves, react with body cells, and release bad substances. The series of these events constitutes the prevalence of several diseases. The word ‘target site’ is very important since the microbe cannot establish themselves in any place in the human body. A clear-cut idea of this concept will be given in the next article.

Infection by a microbe can be best understood through relationships. A relationship is always exhibited between two organisms. There are three different kinds of relationships namely parasitism, commensalism, and symbiosis.

Commensalism– the relationship in which one of the organisms is benefitted and the other is neutral (neither harmed nor benefitted)

Symbiosis/mutualism– the relationship in which both the organisms help out with each other so that both are benefited

Parasitism– the relationship in which one organism is benefitted and the other is harmed. In this scenario, the harmed one is known as the host and the one causing the harm is the parasite.


The reason for which that all microbes don’t cause disease might be that some of them are cannot cause infection meaning that they are not parasitic. Under those circumstances, there are about 1500+ kinds of microorganisms that particularly target and infect human beings. If so, then we would have got around 1500+ diseases and that is not the case. So there is some inner force that protects/guards us against these microbes. This protective force present in our body is known as immunity.

Immunity is defined as the response generated by the local mammalian in response to the infection caused by the pathogenic microbes and their products (toxins).

As mentioned earlier, the microbe has to enter into the body in order to establish itself. In fact, this is the hardest part for the microorganisms and most of them fail to cross this stage. It is this immunity that prevents most of the microbes to enter the body and cause infections. The immunity identifies and destroys these microbes and prevents their infections.

The first step is known as recognition which is an important characteristic of the immune system. It is the ability that the immune system destroys the objects which are foreign or not present in the body (non-self) and do not affect the body cells (self). An error occurs in this and as a result, the immune system is unable to differentiate both and starts to destroy their own body cells. This condition is known as autoimmunity.

A brief introduction to immunity has been provided, the next article discusses the classification of immunity.