Diseases - Types of Diseases and their Symptoms

Over the years, science and technology has improved to such an extent that we have been able to identify the causes, symptoms and cures for various diseases with a high degree of success. And this is supported by considerably long average lifespan and lower mortality rates. But what exactly is a disease?

What is a Disease?

A disease is a condition that deteriorates the normal functioning of the cells, tissues, and organs. Diseases are often thought of as medical conditions that are characterized by their signs and symptoms.

The disease can also be defined as:

A disease is any dangerous divergence from a functional or normal state of an entity. When a person is inflicted with a disease, he exhibits a few symptoms and signs that range from normal to severe depending upon the medical condition. Hence, in order to identify different diseases, the normalcy of an entity needs to be studied and understood as a clear demarcation between disease and disease-free is not always apparent.

The diseases are usually caused by many factors rather than a single cause. When we have a disease, we eventually show some signs, such as headaches, cough, cold, weakness. These signs are referred to as “symptoms.” In almost all diseases, symptoms are shown immediately after having been struck by the disease. However, it varies depending upon the seriousness of the disease.

Today, there are various ways to classify diseases.

Classification of Diseases
Type Explanation Example
Anatomic Classification This type refers to the affected organ or tissue Heart disease
Topographical Classification Further classified into types such as vascular disease, chest disease, gastrointestinal disease, and abdominal diseases. These are then handled by specializations in medicine that follow these topographical classifications An ENT specialist (Ear-Nose-Throat)
A Gastroenterology specialist etc.
Physiological Classification This type includes diseases that affect a process or a function (such as metabolism, digestion or respiration) Diabetes
Pathological Classification This type considers the nature of the disease. For instance, cancer is associated with uncontrolled cell growth, and there are variations or types in the disease. Neoplastic diseases (uncontrolled cell growth that are characteristic of cancer)
Inflammatory diseases (autoimmunity)
Epidemiological Classification This classification refers to the rate of occurrence, distribution and the control of the disease in a population. Epidemic diseases such as plague and Influenza pandemic of 1918–1919

Types of Diseases

Diseases can be caused due to external or internal factors. Immediate causes of diseases can be of two types:

  • Infectious causes of diseases
  • Non-infectious causes of diseases

Infectious And Non-infectious Causes Of Diseases

Infectious diseases are diseases where microorganisms are immediate causes of disease. They cause diseases as they can spread in the community passing the infected pathogens. Non-infectious causes, on the other hand, are diseases that are not usually caused by any microbial activity, they exhibit internal and not external factors. For instance, a few cancers are caused due to genetic abnormalities, high blood pressure can be caused due to a sedentary lifestyle.

Each disease is caused due to different reasons hence treatment for a particular disease too is carried out accordingly. Some can be treated with over-the-counter drugs while other diseases require us to consult a physician.

Read more: Infectious diseases

Infectious Diseases – Infectious Agents

We have seen the classification of different entities based on various characteristics, for simplification, we classify organisms to group them together and study about them as a class. Similarly, diseases are caused by different microorganisms and can be classified as diseases caused by bacteria, fungi, viruses etc. Some diseases are also caused by multicellular organisms such as worms.

Also Read: Harmful Microorganisms

Listed below are few diseases and the disease-causing agents

Name of Pathogen Disease Caused
Leishmania (Protozoa) kala-azar
Staphylococci (Bacteria) Acne
Trypanosoma (Protozoa) Sleeping sickness
Worms Elephantiasis
Viruses Common cold, influenza, AIDS, dengue fever
Bacteria Typhoid, Cholera, Fever

Based on these above classifications, a disease may fall into any number of these classifications.

Acquired Diseases

Such diseases are acquired after birth. They are further grouped into two types, namely:

  • Communicable diseases
  • Non-communicable diseases

Communicable Diseases

Communicable Diseases

Diseases that spread from one person to another are called communicable diseases. They are usually caused by microorganisms called pathogens (fungi, rickettsia, bacteria, viruses, protozoans, worms). When an infected person discharges bodily fluids, pathogens may exit the host and infected a new person (sneezing, coughing etc).

Examples include Cholera, chicken pox, malaria etc.

Non-communicable Diseases

These diseases are caused by pathogens, but other factors such as age, nutritional deficiency, sex of an individual, and lifestyle also influence the disease. Examples include hypertension, diabetes, and cancer. They do not spread to others and they restrain within a person who has contracted them.

Cancer Cells

Cancer is caused when uncontrolled growth occurs in certain tissues of the body. It can be classified depending upon the type of cell that has been initially affected. Tumours consist of the living cell and therefore, in order to survive, it makes a new blood vessel that enables the cancerous cells to receive essential oxygen and nutrients. This process of forming a new blood vessel is called angiogenesis. It can also enable these cancerous cells to spread throughout the body using the circulatory system. This process is termed as an invasion.

Some of the common types of cancer include breast cancer, colon cancer, cervical cancer, ovarian cancer, kidney cancer, liver cancer etc.

Also Read: Communicable Non-Communicable Diseases

Degenerative Diseases

They are mainly caused by the malfunctioning of vital organs in the body due to the deterioration of cells over time. Diseases such as osteoporosis show characteristics of degenerative diseases in the form of increased bone weakness. This increases the risk of bone fractures.

When degeneration happens to the cells of the central nervous system, such as neurons, the condition is termed as a neurodegenerative disorder. Alzheimer’s is a prominent example of this disorder. Degenerative diseases are usually caused by ageing and bodywear. Others are caused by lifestyle choices and some are hereditary.


An allergic reaction arises when the body becomes hypersensitive to certain foreign substances called allergens. This usually happens when the immune system reacts abnormally to any seemingly harmless substances. Common allergens include dust, pollen, animal dander, mites, feathers, latex and also certain food products like nuts and gluten. Peanuts and other nuts have the capability to cause severe allergic reactions that may induce life-threatening conditions such as difficulty in breathing, tissues swelling up and blocking the airways and anaphylaxis shock.

Other common and less life-threatening symptoms include coughing, sneezing, running nose, itchy and red eyes, skin rashes. One of the best examples of this allergic reaction is Asthma. Sometimes, bee stings, and antbites also trigger allergies. Consumption of shellfish and certain medication can induce allergic reactions.

Asthma is a chronic disease, mainly affects the bronchi and bronchioles of the lungs. One of the factors responsible for this is airborne allergens such as pollens or dust. Symptoms include difficulty in breathing, wheezing, and cough.

Deficiency Diseases

They occur due to the deficiencies of hormones, minerals, nutrients, and vitamins. For example, diabetes occurs due to an inability to produce or utilize insulin, goitre is mainly caused by iodine deficiency, kwashiorkor is caused by a lack of proteins in the diet. Vitamin B1 deficiency causes beriberi.

Read more: Deficiency Diseases


It is an abnormal enlargement of the thyroid gland by blocking the oesophagus or other organs of the chest and neck. This causes difficulty in breathing and eating.

Blood Diseases

Blood contains the plasma, white blood cells, platelets and red blood cells. When any of these components are affected, it can lead to blood disorders. For instance, the red blood cells are destroyed when a person contracts the sickle cell disease. The red blood cells are distorted into the shape of a sickle (hence, the name) and it loses its ability to carry oxygen. Consequently, this disease is characterized by symptoms similar to chronic anaemia, such as shortness of breath and tiredness.

Other diseases such as eosinophilic disorders, leukaemia, myeloma (cancer of plasma cells in bone marrow), Sickle Cell Anemia, Aplastic Anemia, Hemochromatosis and Von Miller and Disease (blood-clotting disorder) fall under this classification.

General Symptoms: Pale skin, swelling of lymph nodes, fever, bleeding, bruising, skin rashes, etc.

List of some important diseases and their causative agents.

List of Diseases
Disease Causative Agent
Plague Pasteurella pestis
Cholera Vibrio comma (Vibrio cholera)
Tetanus Clostridium tetani
Anthrax Bacillus anthracis
Whooping cough Bordetella pertussis
Human papillomavirus infection Human papillomavirus
Acquired Immune Deficiency Syndrome (AIDS) Human Immunodeficiency Virus (HIV)
Hepatitis Hepatitis A, Hepatitis B, Hepatitis C, Hepatitis D, Hepatitis E viruses
Chickenpox Varicella zoster virus (VZV)
Meningoencephalitis Naegleria fowleri (amoeba)

To know more about diseases, their types, causes, symptoms and other related topics, please visit @ BYJU’S Biology.

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