You may not have heard of it. But chances are pretty decent that if you're a grown-up, you've had fifth disease.
The oddly named ailment - also known as "slapped cheek syndrome" - is quite common among elementary school aged children. Its signature is the scarlet cheek rash that creates that just-slapped look to which its nickname alludes.
The actual medical term is Erythema infectiosum, which means a contagious rash or redness of the skin. But it is generally known as fifth disease, so-called because it was the fifth of a number of exanthems or rash-causing diseases identified. The preceding ones are, in order: measles, scarlet fever, rubella and Dukes' disease.
Most doctors can spot with ease kids afflicted with fifth disease, at least in the cases where the distinctive cheek rash appears.
"For doctors, when you're looking in the waiting room, you can go 'I'll bet I know what that one's got' just because of that rash," says Dr. Joanne Embree, past-president of the Canadian Pediatric Society and a pediatric infectious diseases specialist at Winnipeg Children's Hospital.
Another reason most doctors know it: fifth disease is a favourite of those who draw up medical exams.
"It makes a great question," Embree says. "It's one of the few things that all the doctors know."
They may know about it and be able to spot it, but there's not much doctors can do when confronted with patients with fifth disease.
There's no vaccine, and not much chance one will be developed. "It would be very hard to justify in terms of the health impact," says Dr. Ronald Gold, former head of infectious diseases at the Hospital for Sick Children in Toronto and author of "Your Child's Best Shot: A Parent's Guide to Vaccination."
There's also no treatment, though fortunately in most cases one isn't needed. Embree says kids who get fifth disease are sluggish and cranky, with a fever and sometimes achy joints. "But generally speaking, they're not necessarily all that sick."
Doctors suggest treating the symptoms such as fever with something like children's Tylenol. If the rash itches, an oatmeal bath should help, says Dr. Vinita Dubey, an associate medical officer at Toronto Public Health.
The flu-like symptoms develop first and that's when people with fifth disease are infectious. By the time the rash shows up, the patient is on the mend - and no longer spreading the virus.
"So classically a child might get a fever, runny nose, kind of mild, cold-like respiratory symptoms. The symptoms will typically resolve and then days later this rash will show up," says Dubey.
That feature - the fact you only know you're dealing with fifth disease after it's had a chance to spread - makes controlling outbreaks impossible.
Public health officials don't even try. In fact, they recommend schools and daycares not exclude children with the condition because by the time fifth disease is recognized, other kids will be incubating the virus, coming down with the symptoms and about to develop the sandpaper-like rash too.
"By the time you start getting a couple of people you look back to see, well, where did they get it from? But by the time they've gotten their symptoms of the rash, they've already spread it because they're done their contagious period," Dubey says.
While the course of the illness is overwhelmingly mild most of the time, in rare cases fifth disease can be dangerous.
The virus that causes fifth disease - parvovirus B19 - temporarily interferes with the production of red blood cells. While most people can handle that, on occasion people can develop anemia and even severe anemia. Gold says anyone with sickle cell anemia or other forms of chronic anemia can become very sick when infected with fifth disease.
The most common place where it causes problems, though, is in pregnancy. A pregnant woman who becomes infected, especially during the early part of her pregnancy, runs a risk her fetus will become severely anemic. In some cases, that anemia can lead to the death of the fetus.
Concern often focuses on elementary school teachers. They are in almost constant contact with the population most likely to become infected with fifth disease - kids.
If there is an outbreak in a school, pregnant teachers can determine if they've ever had the condition by having their blood checked for antibodies. Those who've had it before have no cause for concern; those who haven't may be asked or may choose to go on leave to minimize their exposure.
Even that step may not be of much help, Dubey says.
"The fact that you're seeing it in schools is usually just a sign that it's spreading in the community. So staying home from school doesn't necessarily protect you because you're still taking part in your regular daily activities in the community and that's typically where the transmission is also occurring," she notes.
"The general consolation here is that by and large, most people are immune. So by the time you would be getting pregnant ... most adults would be immune."
Some estimates suggest about half of adults are immune to fifth disease because of prior infection. The virus spreads via respiratory droplets - microscopic drops of saliva or phlegm sneezed or coughed out by those who are infected.
In addition to anemia, the disease causes temporary arthritis in some cases. This inflammation of the joints is more common when people catch fifth disease in adulthood.