Infections are caused by bacteria, fungi, other microorganisms, viruses, and prions. From the beginning of time, infections have been a major cause of illness and death. Powerful infectious diseases that sometimes give rise to epidemics like smallpox, influenza, tuberculosis, and plague have had a major impact on large numbers of people in the world for centuries. Infectious diseases and epidemics have influenced the outcomes of regional conflicts and the socioeconomic development of numerous cities, states, and countries. The types of infections affecting humans include common bacterial diseases such as pneumonia, urinary tract infections, and skin infections as well as viral disorders such as influenza and chicken pox.
Noninfectious diseases include all the medical and surgical conditions that are not mental disorders or infections. This is a broad category that includes muscular and skeletal conditions, cardiovascular disease, autoimmune diseases, kidney diseases, lung diseases, and diseases affecting the gastrointestinal tract. Virtually all areas of common human disease conditions are covered in this reference work, including mental disorders, which are conditions thataffect thinking, behavior, personality, judgment, and brain function. Examples of mental disorders include anxiety, depression, eating disorders, mood disorders, schizophrenia, and personality disorders. Mental disorders are often underdiagnosed and may not be publicly disclosed because of fear of the stigma associated with mental illness or because of a lack of knowledge about its causes, diagnostic methods of detection, or available therapies and treatments. Research has led to great improvements in the scientific knowledge about mental illnesses.
Some diseases that run in families are categorized as genetic diseases; these include depression, diabetes, high blood pressure, and some cancers. In some cases, risk factors and the causes of these diseases are known, however, for many diseases, the exact causes are still unknown. Diabetes, obesity, and cardiovascular disease have been increasing dramatically in the United States and other parts of the world as a result of changes in dietary patterns and reduced aerobic exercise, but these diseases also manifest genetic factors that are the subject of ongoing research.
Bacteria, viruses, and other microorganisms cause infections by penetrating into human or animal organs, tissues, and cells and then replicating to cause disease. Microorganisms can cause disease by damaging and killing human cells, producing toxins, and creating an inflammatory response. Some microorganisms are harmless in normal healthy individuals, but many microorganisms can cause disease if they penetrate the body’s normal host defenses and immune system. Certain organisms that are less virulent and do not normally cause disease are capable of causing opportunistic infections in individuals who have weakened immune systems as a result of organ transplantation, chemotherapy, acquired immunodeficiency syndrome (AIDS), or medications that suppress the immune system.
One of the unique characteristics of infectious diseases is the variety of mechanisms by which these diseases are transmitted or acquired. Some infectious diseases are considered communicable diseases and can be transmitted person to person through direct contact with infected persons or a contaminated environment, large droplets that are shed in close proximity when infected people cough or sneeze, and airborne transmission, especially in contained environments with limited air circulation.
There are also a number of infectious diseases that are transmitted through exchange of body fluids or as sexually transmitted diseases. Some are transmitted as blood-borne pathogens through transfusions or they are spread when people share contaminated needles or when they have unprotected intercourse with infected persons. Still other infections occur through fecal-oral contamination, when food or water become contaminated with bacteria or viruses, or both, causing vomiting and diarrhea. Food-borne illnesses have been increasingly reported worldwide as a result of the globalization of food production and distribution. Infections such as rabies can be acquired from animals; this is called zoonotic transmission.
Noninfectious diseases include all the medical and surgical conditions that are not mental disorders or infections. This is a broad category that includes muscular and skeletal conditions, cardiovascular disease, autoimmune diseases, kidney diseases, lung diseases, and diseases affecting the gastrointestinal tract. Virtually all areas of common human disease conditions are covered in this reference work, including mental disorders, which are conditions thataffect thinking, behavior, personality, judgment, and brain function. Examples of mental disorders include anxiety, depression, eating disorders, mood disorders, schizophrenia, and personality disorders. Mental disorders are often underdiagnosed and may not be publicly disclosed because of fear of the stigma associated with mental illness or because of a lack of knowledge about its causes, diagnostic methods of detection, or available therapies and treatments. Research has led to great improvements in the scientific knowledge about mental illnesses.
Some diseases that run in families are categorized as genetic diseases; these include depression, diabetes, high blood pressure, and some cancers. In some cases, risk factors and the causes of these diseases are known, however, for many diseases, the exact causes are still unknown. Diabetes, obesity, and cardiovascular disease have been increasing dramatically in the United States and other parts of the world as a result of changes in dietary patterns and reduced aerobic exercise, but these diseases also manifest genetic factors that are the subject of ongoing research.
Bacteria, viruses, and other microorganisms cause infections by penetrating into human or animal organs, tissues, and cells and then replicating to cause disease. Microorganisms can cause disease by damaging and killing human cells, producing toxins, and creating an inflammatory response. Some microorganisms are harmless in normal healthy individuals, but many microorganisms can cause disease if they penetrate the body’s normal host defenses and immune system. Certain organisms that are less virulent and do not normally cause disease are capable of causing opportunistic infections in individuals who have weakened immune systems as a result of organ transplantation, chemotherapy, acquired immunodeficiency syndrome (AIDS), or medications that suppress the immune system.
One of the unique characteristics of infectious diseases is the variety of mechanisms by which these diseases are transmitted or acquired. Some infectious diseases are considered communicable diseases and can be transmitted person to person through direct contact with infected persons or a contaminated environment, large droplets that are shed in close proximity when infected people cough or sneeze, and airborne transmission, especially in contained environments with limited air circulation.
There are also a number of infectious diseases that are transmitted through exchange of body fluids or as sexually transmitted diseases. Some are transmitted as blood-borne pathogens through transfusions or they are spread when people share contaminated needles or when they have unprotected intercourse with infected persons. Still other infections occur through fecal-oral contamination, when food or water become contaminated with bacteria or viruses, or both, causing vomiting and diarrhea. Food-borne illnesses have been increasingly reported worldwide as a result of the globalization of food production and distribution. Infections such as rabies can be acquired from animals; this is called zoonotic transmission.