Disease is the term used for any condition or tendency that hampers the normal functioning of the body. In general, human beings refer the term 'disease' to any condition that causes uneasiness, dysfunction etc. The study of diseases is called pathology. Also, the classification of diseases is called Nosology.



Diseases can be classified based on the cause or the mechanism by which the disease is caused.

There are various factors that cause disease. They are classified as:

 Intrinsic factor - Disease caused due to the abnormalities existing within the body. They include genetic defects, stress-related diseases, autoimmune disorders, nutritional defects etc.

 Extrinsic factor - Disease caused due to environmental conditions. An example for extrinsic factor may be passive smoking.

There are also other factors that cause diseases they are social, psychological, chemical and biological. These factors may fall into intrinsic or extrinsic factors. For example, when it comes to cigarette smoking the smoke released contains chemical substances that causes diseases to the smoker as well as the neighboring person. This is called either chemical factor or extrinsic factor.

When harmful germs or microbes (like bacteria, viruses and protozoa) enter the human body, they multiply and cause diseases, the human body is said to have an infection.

Infections are classified as

 Infectious disease

 Non-infectious disease

Infectious diseases are those diseases that transmit from one person to another. These diseases are transmitted through air, insect bites, infected injections, contaminated water or food and sometimes through blood transfusion. These diseases can also be called as contagious or communicable diseases. To avoid spreading of this type of disease it is advised to take the appropriate medicines that fight the germs that cause them. Also, it is better to stay away from person during the time when the disease can be spread.

Non-infectious diseases are those diseases that do not transmit from one person to another. Examples of non-infectious disease are diabetes, stress-related disease etc.

The nine most expensive diseases in the US according to an article in Forbes, based on the research performed by Agency for Health Care Research and Quality (AHRQ) are:

Diseases Annual Cost

Heart Conditions (this excludes hypertension and elevated cholesterol) $68 billion

Trauma $56 billion

Cancer $48 billion

Mental Illness $48 billion

Respiratory Ailments $45 billion

Hypertension $32.5 billion

Arthritis And Joint Disorders $32 billion

Diabetes $28 billion

Back Problems $23 billion

The research further provides with the fast growing diseases in the US. Some of the diseases listed in the research are:

 Esophageal Disorders

The cost per patients has come down whereas the number of people diagnosed with heartburn and related disorders have increased by 509% to 10 million between 1997 and 1996-2002.

 High Cholesterol

The number of people diagnosed with High Cholesterol has increased by 145% to 19 million.

 Hemorrhoids

The number of people increased by only 23% to 1.2 million.

 Anxiety Disorders

The number of patients treated for anxiety has doubled to 12 million over five years.

 Breast Cancer

The number of cases rose 65% to 1.2 million.