Sexually Transmitted Diseases (STDs)

Sexual contact between people is one important way that many diseasecausing organisms spread. More than 30 bacterial, viral, and parasitic diseases can pass from person to person in this way.


A person cannot develop shingles unless he or she has had chickenpox. Shingles is caused by the same virus that causes chickenpox, the herpes zoster virus.


"I studied for days, but I failed the test. I'm in shock," says one teenager to another.

Sickle-cell Anemia

Normally, red blood cells (RBCs) are rounded and flattish, like a saucer. They pass easily through the tiniest blood vessels.


Sinusitis is one of the most common health problems. It affects more than 35 million Americans each year, often because they catch a cold or have allergies.

Skin Cancer

In 1985, former president Ronald Reagan had a growth called a basal (BAY-zuhl) cell carcinoma removed from the side of his nose. The president had often been described as looking tanned and healthy, and when the news broke, it raised public awareness of skin cancer and the dangers of overexposure to the sun.

Skin Conditions

Alison loved to take long walks in the woods in the summer, but one day she developed a streaky rash two days after she had gone on a hike. At first, her skin was red and swollen in spots.

Sleep Apnea

James loved his grandfather, but he was dreading this year's visit. When James shared his room with Grandpa last year, he did not sleep all week.

Sleep Disorders

When people do not get a normal refreshing sleep, they are not at their best. They may be impatient or careless, or they may show poor judgment in the things they do.

Slipped Disk

The spine is made up of bones called vertebrae (VUR-te-bray) that protect the delicate spinal cord. These vertebrae are separated from each other and cushioned by disks.


Beginning in 1966, the World Health Organization declared war against an enemy that throughout history had caused governments to fall and wiped out entire populations. The allies in this battle were mostly ordinary people in villages and towns, and their main weapon was a vaccine.

Speech Disorders

Speech and language develop most intensively during the first three years of life. When babies are born, they can make sounds by pushing air out of the lungs and through the vocal cords in the throat.

Spina Bifida

As part of a sixth grade science project, Brian chose to report on a condition called spina bifida. He showed a picture of the ring-shaped bones, or vertebrae, of the spine and demonstrated how the vertebrae protect the spinal cord and anchor muscles.

Stomach Cancer

The stomach is the sac-like organ located in the upper abdomen, under the ribs, which plays a role in the digestion of food. It connects the esophagus (e-SOF-a-gus), the tube that carries swallowed food, with the small intestine, which absorbs the nutrients needed by the body.


When people cross their eyes, the world suddenly doubles. Images, like the words on this page, become blurred, and it appears as if there are two of everything.

Strains and Sprains

Strains and sprains are injuries to the body's soft tissues—its muscles, tendons, and ligaments. They are everyday occurrences for athletes but can happen to anyone as the result of a fall, a twist, or any other sudden blow to the body.

Strep Throat

In addition to having a sore throat and fever, people with strep throat feel generally weak and tired. They may also have a runny nose, have a headache, or the lymph nodes* in their neck may become enlarged.

Stress-Related Illness

Imagine Alicia, the goalie on the traveling soccer team, with the opposing team barreling toward her with the ball in possession. Imagine Eduardo at 7:59 a.m., running for the school bus that leaves at 8:00 a.m.


While eating lunch with her grandfather on a sunny afternoon, fourteen-year-old Carmen was in the midst of describing her summer camp plans when suddenly one side of her grandfather's face went slack. He tried to speak, but he was slurring his words.

Substance Abuse

Substance abuse is a serious problem in the United States. People who abuse substances can get sick, ruin their relationships with other people, destroy their lives and the lives of family members, and even die.

Sudden Infant Death Syndrome

Mrs. Wyatt is doing all the things her doctor told her to do with her new baby.


It was 1494, not long after Christopher Columbus had returned from his first voyage to the New World. Spanish soldiers, including some of Columbus's men, were fighting the French in Italy.