Human diseases ( SCIENCE )

1. Infectious diseases
A disease caused due to microorganisms is known as an infectious disease.

Infectious agents: They are the disease-causing microorganisms which belong to different categories such as:

Infectious agents	Viruses
	Some bacteria
	Some fungi
	Some unicellular animals
	Some multicellular animals

•	Viruses – They are very tiny organisms which are visible only with the help of electron microscope. They cannot grow, multiply, or reproduce on their own. They need to infect a host cell to get the required machinery to perform these functions. Influenza, cold (Rhinovirus), dengue, AIDS, etc., are certain human diseases caused by viruses.
•	Bacteria – Bacteria are unicellular organisms that are larger than viruses. There are only some bacteria that cause diseases while some other bacteria are useful in nature. Whooping cough, typhoid, cholera, anthrax, etc., are certain human diseases caused by bacteria.
•	Fungi – They are plant-like organisms, which are heterotrophic, i.e., they lack chlorophyll. Their cells have cell walls. Athlete’s foot, candidiasis, ringworms, etc., are some human diseases caused by fungi.
•	Protozoa – They are simple, primitive unicellular organisms which include Amoeba, Trypanosoma, and Leishmania. They are often found in water as they need moisture for survival. Hence, they cause diseases through contaminated water. Amoebiasis, kala azar, malaria, African sleeping sickness, etc., are some of the diseases caused by protozoans.
•	Multicellular animals like worms – Worms are the parasites that infect intestines of human beings and other animals. It includes round worms, pinworms, hookworm, tapeworm etc. Diarrhoea, anaemia, liver rot, etc., are caused by these worms.

2. Deficiency diseases: These diseases are caused by the deficiency of one or the other nutrient in the diet. For example,

i.	Deficiency of protein may cause Kwashiorkar and marasmus
ii.	Deficiency of iron causes anaemia
iii.	Deficiency of iodine causes goitre
iv.	Deficiency of vitamin A causes night blindness
v.	Deficiency of vitamin B1 causes beri-beri