Theory:

What is health?
In 1948, the World Health Organization (WHO) defined health as follows:
Health is a state of complete physical, mental, and social well-being and not merely the absence of disease or infirmity.
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Healthy people carrying different icons related to health
  
Health is wealth. In general, the term health is used to indicate the state of a person being healthy or not.
A person who is free of diseases is known as a healthy person.
The word 'health' is also used in other contexts, such as to denote the facts which are non-living. For example, the health of an organization. In this case, the term health is used to indicate the fact of whether the institution is functioning correctly or not. Thus in simple terms, health refers to the physical, emotional and psychological well-being of a person.
 
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Illustration of health
The harmonious balance between the state of human individual combined with his environment is known as health.
Hygiene:
The good habits and practices that are followed to prevent diseases maintain good health through cleanliness, consumption of safe drinking water, and sewage disposal are known as hygiene. Hence, hygiene refers to all those activities that are performed to improve and maintain good health and mind.
What is sickness?
The state of being ill or unhealthy is known as sickness.
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Illustration shows a sickness
 
The people belonging to different countries or backgrounds explain the causes of sickness in various ways.
Various causes like smell, taste, anxiety, fear, depression and so on are causes of sickness.
Why a baby gets diarrhoea?
Diarrhoea is said to be caused by various causes. The main causes for diarrhoea include infections by microbes, unavailability of good water system or proper sanitation.
 
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Pathogens affecting the large intestine and causes diarrhea
Most people see that the cause of sickness is mostly in terms of their own experience and point of view. All these causes are right or partially right, possibly because each of these causes may be a part of the various reasons due to which a baby gets diarrhoea.
To prevent and treat the sickness successfully, one should completely understand the common sickness in an area and the various combinations of reasons that caused them.
Reference:
https://www.freepik.com/free-vector/healthy-people-carrying-different-icons_3585182.htm
https://www.freepik.com/free-vector/mental-health-concept-illustration_8252004.htm
https://www.freepik.com/free-vector/contemplating-concept-illustration_9930991.htm#&position=4
https://www.freepik.com/free-vector/group-therapy-illustration-concept_9907643.htm
https://www.vecteezy.com/vector-art/171449-free-affliction-illustrarion
https://commons.wikimedia.org/wiki/File:Depiction_of_a_person_suffering_from_Diarrhea.png
https://pixabay.com/it/vectors/anatomia-batteri-batterio-viscere-160524/