Theory:

As we know, the entire diversity observed in the living world has been classified into a few groups based on the common characteristics of different organisms. The organisms that cause diseases are classified.
They are viruses, bacteria, fungi and single-celled animals or protozoans. A few diseases are caused by protozoans and by multicellular organisms, such as worms of different kinds.
Viruses:

These are the infectious agents which cause diseases such as common cold, influenza, dengue fever, chickenpox, mumps, trachoma, measles, poliomyelitis, corona and AIDS. These agents live inside the host (infected organism). The viruses are not related to each other as they do not perform any life process. They multiply very rapidly and hence immediately cause the diseases.
 
Bacteria:
 
These are the agents that cause diseases such as typhoid fever, diphtheria, cholera, anthrax, whooping cough, leprosy, tetany, acne and syphilis. They may or may not live inside in the host. Taxonomically all bacteria are closely related to each other as they have similar life processes. Similar to viruses, they multiply rapidly.
 
Fungi:
 
These are the organisms that are responsible for the skin diseases. Diseases such as ringworm, athlete's foot, skin infection are some of the diseases caused by fungi.
 
Protozoans:
 
These are organisms that cause the familiar diseases such as malaria, sleeping sickness, amoebiasis and kala-azar.
 
Worms:
 
These diseases such as filariasis or elephantiasis, intestinal worm infections and ascariasis are some of the diseases caused by worms. These organisms multiply very slowly when compared to the others.