They are the factors or causes that lead to the disease causing pathology. These Hetu can be internal (nija) and external (agantu) and are capable of disturbing the balance of three doshas (i.e. Vata, pitta and kapha).

In ayurveda Hetu has been classified into different branches that are as follows:

1a) Sannikrisht (close factor)

These are the factors that bypass the sanchay stage of the disease process to the next stage of disease process i.e. prakopa (provocation) by disturbing the equilibrium of the three doshas (vata, pitta, kapha) by the three following factors:

• Stages of the day (morning, afternoon, evening)
  
• Stages of night (early night, mid night, late night)
  
• Stages of digestion (amawasth, pachyamanawastha, pakwastha) An illustrating example of such a disease is trauma (Sannikrisht).

1b) Viprikrisht (distant factor)

These are the factors that effect the equilibrium of doshas, which was developed earlier but was unable to develop disease. In this case as the disturbed equilibrium of dosha (sanchay) already exists the onset of the ailment is quick due to other stimulating factors.

An illustrating example is: during shishirarutu (winter) season equilibrium of kapha dosha is disturbed but it doesn't lead to disease as it is frozen due to extreme cold but as soon as the climate changes it melts due the warmth in wasanth ritu (autumn).

1c) Vyabhichari (weak factor)

This type of Hetu is weak and due to its weakness although it disturbs the equilibrium of the doshas it is unable to lead to disease. An illustrating example is that of our daily life where we resort to unhealthy practices but doesn't always fall sick.

1d) Pradhanik (string factor)

Due to its strength these types of Hetu always lead to disease or death after disturbing the equilibrium of the doshas.
An illustrating example is that of poison, snake bites, trauma etc.

2a) Asatmiya Indriyartha Sanjog

These types of factor are due tot he unhealthy or improper use of the indriya i.e. gyan indriya (chakshu, srotra, ghran, rashna, sparsh ) and karmindriya ( hasta, pad, wak, payu, upasth ) that lead to disease. These Hetu are divided into the following categories:

• Atiyog (overuse)
  This refers to the overuse of indriya. Example: reading for hours together, watching T. V for long and working on computers for hours together develop disease related to eyes due to the overstrain or overuse of eyes.

• Hinyog (underuse)
  This refers to the under use or less use of indriya that lead to disease. Example: if we keep the eyes close for longer than the sleep hours or rest hours eyes may develop some disease.

• Mithyayog (misuse)
  This refers to the wrong use or misuse of indriya. Example: seeing towards bright light straight or reading in very dim light both are misuse of eyes that lead to diseases.

2b) Pragyaparadh

This term comprises of two words i.e. Pragnya meaning intellect and apradh meaning crime thus giving the whole term the meaning of doing wrong things inspite of knowing about their negative side effects or consequences that will lead to disease. In ayurveda it is said that pragyapradh is the main cause of janpadodhwans (epidemic diseases). It is the crime performed by the intellect and is classified into three groups that are as follows:

Dhi (intellectual)

It refers to the onset of any disease due to the action performed by his intellect as he doesn't know what are the adverse effects of the action, lack of intellect.

Example: A person not knowing the bad effect of alcohol resorts to drinking does so because of lack of intellect.

Dhriti (awareness)

This refers to the onset of any disease due to factors or actions performed by the person after knowing the adverse effects of the action i.e. inspite of the awareness when one does wrong things.

Example: when a person knows that alcohol is bad for health but still is addicted to it and is not able to leave it due to liking it or weak will power.

Smruti (memory)

It refers to the onset of ailment due to the loss of memory.

Example: A person who is aware of the bad effects of alcohol decides not to drink in future but drinks forgetting that he had resolved not to drink he is termed as smrutivibhrast.

2c) Parinama (time)

Parinama refers to the prime factors that change the world and is classified under the following three categories:

Hinayog
Less cold in winter is considered as hinayog of winter season, which effects our body and leads to disease.

Mithyayog
If there is unexpected rain in winter is considered as mithyayog of it. It also effects our body and leads to disease.

Atiyog
Extreme cold in winter is considered as atiyog of it. It effects our body, which leads to disease.

3a) Dosha Hetu

This refers to the Hetu that disturbs the equilibrium of any particular dosha but doesn't lead to particular disease.

Example: intake of sugar causes disequilibrium in kapha dosha but doesn't lead to any disease in particular.

3b) Vyadhihetu (diseased factor)

This refers to the cause of disease that is direct and can be identified immediately as the cause.

Example: eating soil always leads to anemia.

3c) Ubhayahetu (combined factor)

It is the situation when both the disease and disturbance of thed oshas is identified with the same factor or cause.

Example: Eating chilies or spicy food during travelling by any vehicle or any other means of transport leads to disease called vatarakt ( gout). The dosha disturbed in this case is vata.

4a) Utpadakhetu (Predisposing Factor )

This refers to the situation where the factor is responsible for disturbing the equilibrium of the dosha but may or my not be responsible for the disease.

Example: Coldness of winter disturbs the equilibrium of kapha dosha in the body but it does not lead to any disease.

4b) Vyanjakhetu (exciting factor)

This refers to the factors, which are not responsible for disturbing the dosha but exciting them to lead to the disease.

Example: Warmth of vasant (autumn) does not disturb the equilibrium of kapha dosha directly but its warmth melts the disturbed kapha dosha, which leads to disease.

5a) Bahyahetu (external factor)

This refers to the disease causing factors that are present outside the body.

Example: Diet, conduct, time, microorganism, trauma, insect bite, lightening, poisons, etc.

5b) Abhyantarhetu (interanl factors)

This refers to the factors present inside the body.

Example: Dosha, Dhatu and Mala of the body.