Sometimes the immune system overreacts to a particular substance called an allergen, producing an allergy. This may result from eating certain foods, inhaling particles or exposure of the skin to chemicals. Common allergies include asthma, hay fever, urticaria, farmer’s lung, food allergies and contact dermatitis. Treatments include avoiding allergens and antihistamine drugs. In some cases anaphylactic shock the body reacts so violently to an allergen, such as a wasp sting, that without treatment the patient may die.
The body’s tissue cells constantly divide and replace themselves. Some times “rouge” cells divide out of control to produce an abnormal tissue growth or malignant tumor. Diseases involving these tumors are called cancers. If untreated, cancer cells spread from the tumour via the blood stream to other parts of the body where they produce secondary tumours. Eventually, cancer cells overwhelm the body and the patient dies. The risk of developing a cancer may be inherited, or it may be increased by smoking, drinking alcohol or exposure to chemicals. Cancer affects around 25 percent of people in the western world at some point in their lives and is the second most common cause of death after heart disease.
Different types of cancers do not occur with the same frequency worldwide. This uneven distribution points to the involvement of environmental factors- such as food intake, smoking or exposure to ultra violet light- as causes of cancer. For example, stomach cancer in Japan and Chile is believed to be related to diets high in salted and pickled food.
Infections arise when disease causing micro organisms penetrate the body’s physical defenses such as the skin and entire bloodstream and tissues near their route of entry can be by infection of infected droplets, ingestion of contaminated food or water, entry through broken skin, skin to skin contact, injection by needle or other sharp object, sexual contact, insect bites, or by transmission from mother to fetus. In most of the case the body immune system acts to destroy the invading micro organism before it can do harm. Most disease signs and symptoms such as spots in measles- are indicators of the battle between the micro organism and the body’s immune system.
In rare cases, the infection may be so virulent that kills the patient before the immune system has time to fight back. Before the 20th century infectious diseases were a major cause of deaths in the world. In developed countries today, deaths from infectious disease have been dramatically reduced, and non infectious diseases such as cancers and heart disease have taken over as major killers. This reduction has been achieved by better public health and sanitation; the use of antibiotics and other antimicrobial drugs; and the use of vaccines.
However there is no room for complacency since old diseases such as TB, malaria are again on the increase, some bacteria are now resistant to drugs; many viral conditions are untreatable and new diseases such as AIDS are appearing. Disease causing organisms include viruses, with are non living infectious agents; single celled organisms including bacteria, chlamydiae, rickettsiae, fungi and protists, and multicellular organisms such as parasitic worms.
Viruses consist of genetic material surrounded by a protein coat, and cause diseases such as measles, mumps and rabies. Viruses have to invade host cells to reproduce. They lose their outer protein coat and use the host cell’s DNA to replicate their genetic materials. A protein coat is constructed around the rebuilt genetic material and newly formed viruses burst out of the host cell or exit in an envelope and invade other host cells to multiply further.
Non Infectious Diseases
Non infectious diseases are now the major cause of death in the developed world. The reason for this is that many infectious diseases are now treatable with drugs. Also the incidence of non infectious diseases increases with age, and averaged life expectancy is increasing. Non infectious diseases include circulatory diseases including heart attacks and strokes, in which the blood vessels to the heart or brain becomes blocked; cancers diabetes mellitus, which is the body’s inability to control blood sugar levels; nervous system diseases such as motor neuron disease; respiratory system diseases digestive system diseases ; kidney diseases and allergies. On average, infectious diseases are responsible for just five deaths per 100000 of the population per year in the developed world, as compared with nearly 1000 deaths per 100000 caused by major non infectious diseases.