Cancer affects our cells, the body's basic unit of life. To understand cancer, it is helpful to know what happens when normal cells become cancerous.
The body is made up of many types of cells. Normally, cells grow, divide, and produce more cells as they are needed to keep the body healthy and functioning properly. Sometimes, however, the process goes astray -- cells keep dividing when new cells are not needed. The mass of extra cells forms a growth or tumor. Tumors can be either benign or malignant.
Benign tumors are not cancer. They often can be removed and, in most cases, they do not come back. Cells in benign tumors do not spread to other parts of the body. Most important, benign tumors are rarely a threat to life.
Malignant tumors are cancer. Cells in malignant tumors are abnormal and divide without control or order. These cancer cells can invade and destroy the tissue around them. Cancer cells can also break away from a malignant tumor. They may enter the bloodstream or lymphatic system (the tissues and organs that produce and store cells that fight infection and disease). This process, called metastasis, is how cancer spreads from the original (primary) tumor to form new (secondary) tumors in other parts of the body.
The Colon and Rectum
The colon and rectum are parts of the body's digestive system, which removes nutrients from food and stores waste until it passes out of the body. Together, the colon and rectum form a long, muscular tube called the large intestine (also called the large bowel). The colon is the first 6 feet of the large intestine, and the rectum is the last 8 to 10 inches.
Understanding Colorectal Cancer
Cancer that begins in the colon is called colon cancer, and cancer that begins in the rectum is called rectal cancer. Cancers affecting either of these organs may also be called colorectal cancer.
Colorectal Cancer: Who's at Risk?
The exact causes of colorectal cancer are not known. However, studies show that the following risk factors increase a person's chances of developing colorectal cancer:
Age. Colorectal cancer is more likely to occur as people get older. This disease is more common in people over the age of 50. However, colorectal cancer can occur at younger ages, even, in rare cases, in the teens.
Diet. Colorectal cancer seems to be associated with diets that are high in fat and calories and low in fiber. Researchers are exploring how these and other dietary factors play a role in the development of colorectal cancer.
Polyps. Polyps are benign growths on the inner wall of the colon and rectum. They are fairly common in people over age 50. Some types of polyps increase a person's risk of developing colorectal cancer.
A rare, inherited condition, called familial polyposis, causes hundreds of polyps to form in the colon and rectum. Unless this condition is treated, familial polyposis is almost certain to lead to colorectal cancer.
Personal medical history. Research shows that women with a history of cancer of the ovary, uterus, or breast have a somewhat increased chance of developing colorectal cancer. Also, a person who has already had colorectal cancer may develop this disease a second time.
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Family medical history. First-degree relatives (parents, siblings, children) of a person who has had colorectal cancer are somewhat more likely to develop this type of cancer themselves, especially if the relative had the cancer at a young age. If many family members have had colorectal cancer, the chances increase even more.
Information about Colds
Sneezing, scratchy throat, runny nose--everyone knows the first signs of a cold, probably the most common illness known to man. Although the common cold is usually mild, with symptoms lasting a week or less, it is a leading cause of doctor visits and of school and job absenteeism.
Scientists supported by the National Institute of Allergy and Infectious Diseases (NIAID) have made significant advances in understanding the structure and disease-causing mechanisms of the many viruses that can cause the common cold, with the goal of preventing and treating this troublesome and costly ailment.
The Problem
In the course of a year, individuals in the United States suffer 1 billion colds, according to some estimates.
Colds are most prevalent among children, and seem to be related to youngsters' relative lack of resistance to infection and to contacts with other children in day-care centers and schools. Children have about six to eight colds a year. In families with children in school, the number of colds per child can be as high as 12 a year.
Adults average about two to four colds a year, although the range varies widely. Women, especially those aged 20 to 30 years, have more colds than men, possibly because of their closer contact with children. On average, individuals older than 60 have fewer than one cold a year.
The economic impact of the common cold is enormous. The National Center for Health Statistics (NCHS) estimates that, in 1992, 65 million cases of the common cold in the United States required medical attention or resulted in restricted activity. In 1992, colds caused 157 million days of restricted activity and 15 million days lost from work, according to the NCHS.
The Causes
The Viruses. More than 200 different viruses are known to cause the symptoms of the common cold. Some, such as the rhinoviruses, seldom produce serious illnesses. Others, such as parainfluenza and respiratory syncytial virus, produce mild infections in adults but can precipitate severe lower respiratory infections in young children.
Rhinoviruses (from the Greek rhin, meaning "nose") cause an estimated 30 to 35 percent of all adult colds, and are most active in early fall, spring and summer. More than 110 distinct rhinovirus types have been identified. These agents grow best at temperatures of 33� Celsius [about 91� Fahrenheit (F)], the temperature of the human nasal mucosa.
Coronaviruses are believed to cause 10 to 20 percent of all adult colds. They induce colds primarily in the winter and early spring. Of the more than 30 isolated strains, three or four infect humans. The importance of coronaviruses as causative agents is hard to assess because, unlike rhinoviruses, they are difficult to grow in the laboratory.
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Approximately 10 to 15 percent of adult colds are caused by viruses also responsible for other, more severe illnesses: adenoviruses, coxsackieviruses, echoviruses, orthomyxoviruses (including influenza A and B viruses), paramyxoviruses (including several parainfluenza viruses), respiratory syncytial virus and enteroviruses.
The causes of 30 to 50 percent of adult colds, presumed to be viral, remain unidentified.
The same viruses that produce colds in adults appear to cause colds in children. However, the relative importance of various viruses in pediatric colds is unclear because of the difficulty in isolating the precise cause of symptoms in studies of children with colds.
Does cold cause a cold? Although many people are convinced that a cold results from exposure to cold weather, or from getting chilled or overheated, NIAID grantees have found that these conditions have little or no effect on the development or severity of a cold. Nor is susceptibility apparently related to factors such as exercise, diet or enlarged tonsils or adenoids.
On the other hand, research suggests that psychological stress, allergic disorders affecting the nasal passages or pharynx, and menstrual cycles may have an impact on a person's susceptibility to colds. For example, NIAID-funded experiments showed individuals under high levels of psychological stress are more prone to infection with any of five cold-producing viruses and more apt to experience respiratory symptoms than people experiencing less stress.
The Cold Season
In the United States, most colds occur during the fall and winter. Beginning in late August or early September, the incidence of colds increases slowly for a few weeks and remains high until March or April, when it declines. The seasonal variation may relate to the opening of schools and to cold weather, which prompt people to spend more time indoors and increase the chances that viruses will spread from person to person.
Seasonal changes in relative humidity may also affect the prevalence of colds. The most common cold-causing viruses survive better when humidity is low--the colder months of the year. Cold weather also may make the nasal passages' lining drier and more vulnerable to viral infection.
Cold Symptoms
Symptoms of the common cold usually begin two to three days after infection and often include nasal discharge, obstruction of nasal breathing, swelling of the sinus membranes, sneezing, sore throat, cough and headache. Fever is usually slight but can climb to 102� F among infants and young children. Cold symptoms can last from two to 14 days, but two-thirds of people recover in a week. If symptoms occur often or last much longer than two weeks, they may be the result of an allergy rather than a cold.
Colds occasionally can lead to secondary bacterial infections of the middle ear or sinuses, requiring treatment with antibiotics. High fever, significantly swollen glands, severe facial pain in the sinuses, and a cough that produces mucus may indicate a complication or more serious illness requiring a doctor's attention.
