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, a few 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.
Virus:
These are the infectious agents which cause diseases such as common cold, influenza, dengue fever, chickenpox, mumps, trachoma, measles, poliomyelitis, corona and AIDS.
 
SAG_Dengue-Virus_160413_01.jpg
DEN-1, 2 virus
 
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 the typhoid fever, diphtheria, cholera, anthrax, whooping cough, leprosy, tetany, acne and syphilis.
 
bacteria-62993_1920.jpg
E.coli bacteria
 
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 the viruses they multiply rapidly.
Fungi:
These are the organisms that are responsible for most of the skin diseases.
 
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A few fungi causes infections
 
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.
 
Protozoa_collage_2.jpg
Some of the disease causing protozoans
Worms:
These diseases such as filariasis or elephantiasis, intestinal worm infections by tapeworms and ascariasis are some of the diseases caused by worms. These organisms multiply very slowly when compared to the others.
 
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A few disease causing worms
Reference:
https://pixabay.com/photos/bacteria-disease-escherichia-coli-62993/
https://upload.wikimedia.org/wikipedia/commons/7/7c/Protozoa_collage_2.jpg
https://upload.wikimedia.org/wikipedia/commons/9/98/SAG_Dengue-Virus_160413_01.jpg
https://upload.wikimedia.org/wikipedia/commons/7/7c/Protozoa_collage_2.jpg
https://commons.wikimedia.org/wiki/File:Tapeworm_SEM2.jpg
https://commons.wikimedia.org/wiki/File:Adult_Loa_Loa_Parasite_(8905176106).jpg
https://www.flickr.com/photos/gtzecosan/15701719491
By Roberto J. Galindo - Own Work: Foto tomada por Roberto J. Galindo, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=2216925
https://upload.wikimedia.org/wikipedia/commons/thumb/7/7e/Athlete%27s_Foot_Fungus.jpg/1024px-Athlete%27s_Foot_Fungus.jpg