There are other non communicable diseases which are not caused by the malfunctioning of organs, but can damage their functioning. Some of these are as follows.

Cancer

This disease develops when the cells of a particular tissue of the body divide at an unnaturally fast rate. The growing mass of cells is called a tumor. Tumors which do not continue growing are called benign tumors. Others, called malignant tumors, grow rapidly and spread to other areas. These ultimately obstruct the functioning of one or more vital organs or system of the body.

A cancerous growth may develop at any age and in many different parts of the body, including the lungs, brain, skin, tongue, bone marrow, and reproductive and digestive organs. The exact cause of cancer is not known. However, certain factors, like smoking, and frequent exposure to certain chemicals and radiation, are known to increase the risk of developing cancer. Cancer is a deadly disease, but can often be kept in check or cured completely if detected in the initial stage.

Allergy

An allergy is an unusual reaction to a particular substance. You might have heard of hay fever, for example. This is caused by hypersensitivity of the eyes and lining of the nasal passage to certain types of pollen grain and dust, resulting in sneezing and watering of the eyes and nose.

Substance causes allergies that are called allergens. Different people are allergic to different substances, and many people never develop an allergy at all. A few common allergens are dust, spores, pollen, certain cloths and wool, particular medicines, animal fur cosmetics and shellfish.

The areas most frequently affected by allergies are skin and the respiratory and digestive tracts. Asthma is a common allergic reaction, as is eczema, which affects the skin. Skin allergies generally cause itching and rashes, while hypersensitivity to particular foods or medicine may lead to nausea, diarrhea, vomiting, and so on. Inflammation and headache are also common symptoms of allergy. The person should try to identify the substance causing the symptoms and avoid exposure to it.

Communicable diseases are caused mostly by microorganisms, commonly called germs. These enter a person’s body in different ways, such as through the air, water or food. They travel to specific tissues, where they multiply and may destroy cell. The poisonous substances, called toxins, released by them affect the normal functioning of that particular part of the body and cause generalized symptoms like fever. A person who has such a disease is said o be infected. These diseases spread from an infected person to a healthy person through direct or indirect contact. Hence, they are called infectious or communicable diseases. Some organisms that cause communicable diseases are as follows.

Bacteria

Many bacteria are beneficial. However, there are few types which cause a variety of diseases among human beings, animals and plants. Some of the bacterial diseases we are prone to are tuberculosis, typhoid, diphtheria, tetanus, pneumonia and cholera. Dysentery can be caused by certain bacteria and so can food poisoning.

Viruses

Unlike bacteria and fungi, there are no beneficial viruses. Viruses can live only as parasites within living cells. When a virus enters the host’s cell, it takes over the cell by releasing its genetic material into it. The cell thus starts reproducing more copies of the virus.

Each virus infects a specific host. So, one which infects a particular species of mammals does not usually infect another species. There are a few exceptions, though, like the rabies virus, which infects dogs, human beings and other animals. Some viral diseases of human beings are smallpox, chickenpox, polio, measles, mumps, influenza, hepatitis and AIDS. Viruses are named after the diseases they cause.

Fungi

Fungi do not cause as many diseases as do bacteria and viruses. There are a few types which afflict animals and plants. In human beings, fungi mostly cause skin diseases, like ringworm and athlete’s foot. Some fungi which grow on food cause serious food poisoning.

Protozoa you already know that protozoan responsible for diseases like amoebic dysentery and malaria. Among the other diseases caused by protozoan are giardiasis, which affects the digestive tract, and sleeping sickness.

Parasitic worms

Tapeworms, roundworms, hookworms and pinworms live as parasites in the intestines o human beings and other animals. Generally, they cause stomach-related problems, but some, like tapeworms, can lead to serious complications. Tapeworms need two hosts to complete their life cycle. Their larvae pass into our intestine from pigs or cows if we eat undercooked pork or beef. They mature into adults and lay eggs, which are passed out with faces. When people defecate in the open, the eggs can be eaten by pigs and cows, and the entire cycle repeated infection by pinworms and roundworms is common among human beings. They spread from one person to another through contact with anything contaminated with their eggs. Practicing proper hygiene can check such contamination. Filarial, or elephantiasis, is another disease caused by a worm.