Rabies is a deadly virus spread to people from the saliva of infected animals. The rabies virus is usually transmitted through a bite.

Animals most likely to transmit rabies in the United States include bats, coyotes, foxes, raccoons and skunks. In developing countries of Africa and Southeast Asia, stray dogs are the most likely to spread rabies to people.

Radiation sickness is damage to your body caused by a large dose of radiation often received over a short period of time (acute). The amount of radiation absorbed by the body — the absorbed dose — determines how sick you'll be.

Radiation sickness is also called acute radiation sickness, acute radiation syndrome or radiation poisoning. Common exposures to low-dose radiation, such as X-ray or CT examinations, don't cause radiation sickness.

Although radiation sickness is serious and often fatal, it's rare. Since the atomic bombings of Hiroshima and Nagasaki, Japan, during World War II, most cases of radiation sickness have occurred after nuclear industrial accidents such as the 1986 fire that damaged the nuclear power plant at Chernobyl or the 2011 earthquake that damaged the nuclear power plant on the east coast of Japan.

Sometimes the terms "reactive airway disease" and "asthma" are used interchangeably. Often, the term "reactive airway disease" is used when asthma is suspected, but not yet confirmed.

Reactive airway disease in children is a general term that doesn't indicate a specific diagnosis. It may be used to describe a history of coughing, wheezing or shortness of breath triggered by infection. These signs and symptoms may or may not be caused by asthma.

Describing a condition as reactive airway disease in part reflects the difficulty in establishing a diagnosis of asthma in certain situations — such as during early childhood. Although it's possible for infants and toddlers to have asthma, tests to diagnose asthma generally aren't accurate before age 6.

Reactive arthritis is joint pain and swelling triggered by an infection in another part of your body — most often your intestines, genitals or urinary tract.

The joints in your knees, ankles and feet are the usual targets of reactive arthritis. Inflammation also may affect your eyes, skin and urethra when you have reactive arthritis.

Although reactive arthritis is sometimes called Reiter's syndrome, Reiter's is actually a specific type of reactive arthritis. In Reiter's, inflammation typically affects the eyes and urethra, as well as your joints.

Reactive arthritis isn't common. For most people, signs and symptoms of reactive arthritis come and go, eventually disappearing within 12 months.

Reactive attachment disorder is a rare but serious condition in which infants and young children don't establish healthy bonds with parents or caregivers.

A child with reactive attachment disorder is typically neglected, abused or orphaned. Reactive attachment disorder develops because the child's basic needs for comfort, affection and nurturing aren't met and loving, caring attachments with others are never established. This may permanently change the child's growing brain, hurting the ability to establish future relationships.

Reactive attachment disorder is a lifelong condition, but with treatment children can develop more stable and healthy relationships with caregivers and others. Safe and proven treatments for reactive attachment disorder include psychological counseling and parent or caregiver education.

Recurrent breast cancer is breast cancer that comes back after initial treatment. Although treatment is aimed at eliminating all cancer cells, a few may survive. These undetected cancer cells multiply, becoming recurrent breast cancer.

Recurrent breast cancer may occur months or years after your initial treatment. The cancer may come back in the same place as the original tumor, known as local recurrence. Or it may spread to other areas, typically your bones, liver or lungs, known as distant recurrence.

Learning you have recurrent breast cancer may be harder than dealing with the initial diagnosis. But having recurrent breast cancer is far from hopeless. Treatment may eliminate local, regional or distant recurrent breast cancer. Even if a cure isn't possible, treatment may control the disease.

Retinal detachment describes an emergency situation in which a critical layer of tissue (the retina) at the back of the eye pulls away from the layer of blood vessels that provides it with oxygen and nourishment.

Retinal detachment leaves the retinal cells lacking oxygen. The longer retinal detachment goes untreated, the greater your risk of permanent vision loss in the affected eye.

Fortunately, retinal detachment often has symptoms that are clear warning signs. Early diagnosis and treatment of retinal detachment can save your vision. If you suspect you may have a retinal detachment, contact an eye specialist (ophthalmologist) as soon as warning signs appear.

A retractile testicle is a testicle that may move back and forth between the scrotum and the groin. When the retractile testicle is residing in the groin, it may be easily guided by hand into its proper position in the scrotum — the bag of skin hanging behind the penis — during a physical examination.

For most boys, the problem of a retractile testicle goes away sometime before or during puberty, the time when an out-of-place testicle moves to its correct location in the scrotum and stays there permanently.

About a quarter of the time, the retractile testicle stays up in the groin and is no longer movable. When this happens, the condition is called an ascending testicle.

Rubella, also called German measles or three-day measles, is a contagious viral infection best known by its distinctive red rash.

Rubella is not the same as measles (rubeola), though the two illnesses do share some characteristics, including the red rash. However, rubella is caused by a different virus than measles and is neither as infectious nor usually as severe as measles.

The measles-mumps-rubella (MMR) vaccine, usually given to children in the United States twice before they reach school age, is highly effective in preventing rubella. Because of widespread use of the vaccine, the Centers for Disease Control and Prevention (CDC) has declared rubella eliminated in the United States, but cautions parents to make sure their children are vaccinated to prevent its reemergence.