2 Notes

“A disease is a condition that makes cells, tissues, and organs work less well than they should.”

People usually think of diseases as health conditions that can be recognised by their signs and symptoms.

Another way to describe the disease is:

“Any dangerous change from an entity’s normal or functional state.”

When a person gets sick, he or she shows a number of symptoms and signs that can range from mild to severe, depending on the illness. So, to figure out what diseases are, you need to study and understand what is normal about an entity. There isn’t always a clear line between diseased and disease-free.

Most of the time, the diseases are caused by more than one thing. When we get sick, we show signs like headaches, coughing, a cold, or feeling weak. “Symptoms” is the word for these signs. Almost all diseases have symptoms that show up right away after someone gets sick. But it depends on how bad the disease is.

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 is 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 the plague and Influenza pandemic of 1918–1919

Types of Diseases

Diseases can be of two types

  1. Infectious diseases
  2. Non-infectious diseases

Infectious Diseases

Communicable diseases are those that can be passed from one person to another. Most of the time, microorganisms called pathogens are to blame (fungi, rickettsia, bacteria, viruses, protozoans, and worms). When a person with an infection pees or poop, pathogens can leave the host and infect a new person (sneezing, coughing etc). Cholera, chickenpox, malaria, and other diseases are some examples.

Non-infectious diseases

Pathogens are what cause these diseases, but age, a lack of nutrients, a person’s gender, and their way of life can also affect the disease. Some examples are high blood pressure, diabetes, and cancer. They don’t spread to other people, and once someone has them, they stay in their body. Alzheimer’s disease, asthma, cataracts, and heart disease are all examples of other diseases that are not caused by germs.

Degenerative Diseases

Most of the time, they are caused by cells breaking down over time, which makes important organs in the body stop working right. Diseases like osteoporosis, which makes bones weaker, are examples of degenerative diseases. This makes it more likely that bones will break.

A neurodegenerative disorder is a condition in which the cells of the central nervous system, such as neurons, break down. Alzheimer’s is a well-known case of this condition. Most degenerative diseases are caused by ageing and wear and tear on the body. Some are caused by the way people live, while others are inherited.


An allergic reaction happens when the body becomes overly sensitive to allergens, which are foreign substances. This usually happens when the immune system has an unusual reaction to something that seems harmless. Dust, pollen, animal dander, mites, feathers, latex, and even some foods like nuts and gluten can cause allergies. Peanuts and other nuts can cause severe allergic reactions that can be life-threatening, such as trouble breathing, swollen tissues that block the airways, and anaphylaxis shock.

Other common, less dangerous symptoms include coughing, sneezing, a runny nose, itchy, red eyes, and rashes on the skin. Asthma is a good example of this kind of allergic reaction. Sometimes, allergies can also be caused by bee stings and ant bites. Some allergic reactions can be caused by eating shellfish or taking certain medicines.

Asthma is a long-term disease that mostly affects the bronchi and bronchioles of the lungs. One reason for this is that allergens like pollen and dust are in the air. Some of the signs are trouble breathing, wheezing, and coughing.

Blood Diseases

Plasma, white blood cells, platelets, and red blood cells are all parts of blood. When any of these things go wrong, it can lead to problems with the blood. For example, when a person gets sickle cell disease, red blood cells are killed. The red blood cells change shape to look like a sickle, which is how the disease got its name, and they lose their ability to carry oxygen. Because of this, this disease has symptoms like those of chronic anaemia, such as shortness of breath and feeling tired.

This group also includes diseases like eosinophilic disorders, leukaemia, myeloma (cancer of plasma cells in the bone marrow), Sickle Cell Anemia, Aplastic Anemia, Hemochromatosis, and Von Miller and Disease, which is a problem with how the blood clots.

Symptoms in general include pale skin, swollen lymph nodes, fever, bleeding, bruising, rashes, etc.

Deficiency Diseases

They happen when there aren’t enough hormones, minerals, nutrients, or vitamins in the body. For instance, diabetes happens when the body can’t make or use insulin, goitre is mostly caused by not getting enough iodine, and kwashiorkor is caused by not getting enough protein in the diet. Beriberi is caused by a lack of vitamin B1.

Disease-Causing 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)

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