Streptococcus

Streptococcus, genus of spherical, gram-positive, aerobic bacteria. The streptococci occur in pairs or chains, and some species are pathogenic in humans. Streptococcal infections include strep throat, scarlet fever, erysipelas, puerperal fever, and some pneumonias. The drugs of choice for treating such infections are penicillin and erythromycin. Cultures of nonpathogenic lactic streptococci are used in the fermentation of dairy products such as cheese and buttermilk.

Smallpox

Smallpox, highly contagious viral disease that is often fatal. The disease is chiefly characterized by a skin rash that develops on the face, chest, back, and limbs. Over the course of a week the rash develops into pustular (pus-filled) pimples resembling boils. In extreme cases the pustular pimples run together—usually an indication of a fatal infection. Death may result from a secondary bacterial infection of the pustules, from cell damage caused by the viral infection, or from heart attack or shock. In the latter stages of nonfatal cases, smallpox pustules become crusted, often leaving the survivor with permanent, pitted scars.

Smallpox is caused by a virus. An infected person spreads virus particles into the air in the form of tiny droplets emitted from the mouth by speaking, coughing, or simply breathing. The virus can then infect anyone who inhales the droplets. By this means, smallpox can spread extremely rapidly from person to person.

Sickle-Cell Anemia

Sickle-Cell Anemia, genetic disorder of the blood leading to frequent and severe infections, damage to major organs, and episodes of unpredictable pain in the back, chest, abdomen, and extremities. Early symptoms appear at about six months of age and may include serious infections, pain and swelling in the hands and feet, and enlargement of the abdomen and heart.

Sickle-cell anemia is caused by a defective gene that produces an abnormal form of hemoglobin, the component of red blood cells responsible for transporting oxygen from the lungs to the tissues. The abnormal hemoglobin, called hemoglobin S, distorts red blood cells after they release oxygen in the tissues. These distorted cells are called sickled cells because of their resemblance to the sickle, a type of crescent-shaped cutting blade used in agriculture. The sickled shape makes it difficult for these cells to pass through tiny blood vessels, resulting in intensely painful blockages that prevent vital oxygen and nutrients in the blood from reaching organs and tissues, impairing their function. As a result, sickle-cell patients are also vulnerable to a number of infections. When the blood flow to the brain is affected, sickle-cell patients may experience brain damage, such as stroke. Sickle cells also break apart more readily than healthy red blood cells, leading to a deficiency of red blood cells, known as anemia.

Sickle-cell anemia is an autosomal recessive genetic disease—that is, a child with the disease must inherit one copy of the defective hemoglobin gene from each parent. Many people carry one hemoglobin S gene with no significant health problems as a result. But these carriers of the sickle-cell gene can pass the gene to their children. When both parents are carriers, they have a one in four chance with every pregnancy of having a child with sickle-cell anemia.

Scurvy

Scurvy, disease of human beings caused by a prolonged deficiency of vitamin C, or ascorbic acid, in the diet. It is characterized by progressive bodily weakness, spongy and inflamed gums, loose teeth, swollen and tender joints, and a tendency toward ecchymosis, or absorption by the tissues of blood from ruptured blood vessels. Anemia often occurs as a direct result of such hemorrhage. Lack of vitamin C in the diet prevents the formation of connective tissues that hold the body's structures together, such as tendons, ligaments, bone, dentin, and cartilage. The disease may appear in adults after about six months of complete lack of vitamin C.

Schistosomiasis

Schistosomiasis or Bilharziasis, widespread disease caused by the infestation of the human body by (flukes) commonly called blood flukes, of the genus Schistosoma. Blood flukes spend most of their life cycle in two hosts; the adult stage is spent in a mammal, usually humans, and the immature stages are spent in certain snails. Eggs discharged from the host hatch into larval forms in fresh water; from the water, the larvae, miracidia, invade the snail that acts as an intermediate host. The larval form of the parasite undergoes partial maturation in the snail, then escapes back into the water, as mature larvae called cercariae. At this stage they penetrate the skin of the host from the water and then migrate through the blood vessels to specific capillaries as maturation completes. There they remain and lay eggs.