Fifth disease is the name given to a common and mild childhood illness.

But the virus that causes the disease can pose a serious, and even fatal, risk to the unborn child of a pregnant woman or to people with some forms of anemia or impaired immune systems."Parvovirus B-19 is probably spread either by airborne droplets that are breathed in, or by direct contact with body fluids containing the virus," said Dr. Keith Krasinski, a pediatrician and specialist in childhood infectious disease at New York University Medical Center.

Fifth disease is medically termed erythema infectiosum, or redness caused by infection. Its symptoms include muscle ache, low-grade fever, and a characteristic rash on the cheeks that resembles a slapped face, and which may spread to the arms, legs and torso.

Outbreaks generally occur in the spring. The illness is usually mild, needs no treatment and runs its course within a week. Symptoms may recur for several weeks and may be made worse by sunlight, heat, or physical or emotional stress.

However, the recently discovered human parvovirus B-19 can inhibit the production of red blood cells, causing a red blood cell deficiency in people with chronic hemolytic anemia or compromised immune systems.

In a developing fetus, whose immune system is immature, the virus can cause chronic infections and anemia.

Infection in adults, especially women, may also lead to joint pain in the hands, wrists, knees and ankles. This usually resolves in a few weeks but could last for months and even years.

While it is advisable to keep people at risk away from a child with fifth disease, this can be difficult, Krasinski noted. Diagnosis may not be evident because fever and muscle ache occur before the rash and seem to indicate a cold or flu. The rash may appear a week after infection, by which time a child is no longer contagious.

People who work at day-care centers and schools should exercise caution to prevent the spread of parvovirus B-19. "Scrupulous hand washing is important, as is wearing gloves when coming in contact with anyone's body fluids," the NYU pediatrician advised.

About 60 percent of all adults are thought to have antibodies against the virus, and these may also protect the fetuses of immune mothers. Tests to see if expectant mothers are immune are generally performed only for women with chronic hemolytic anemia.

Pregnant women who have been exposed to human parvovirus B-19 may wish to be monitored for any signs of fetal abnormality.

Experimental therapy with interuterine transfusions and antibody treatment may prove an effective option in treating the infection in fetuses, but further studies must be conducted, Krasinski said.