The flu is here.

The arrival of the microscopic world traveler, several weeks later than normal for the Washington area, was confirmed yesterday in the laboratories of Children's Hospital National Medical Center.

Adults and children throughout the area have been calling schools and offices since late fall to report that they were sick in bed with "the flu."

They may well have been sick, but only now can anyone with fever, aches, chills, cough and headache claim to have the real thing -- flu.

The virus isolated yesterday at Children's Hospital is a Type B influenza, but scientists there have not pinpointed the strain more precisely than that. They do not know, for instance, from which country it comes.

Type B flu causes national epidemics about every five years. It is not as virulent as Type A, which spawns national epidemics about every three years and worldwide pandemics about every 10. But it is stronger than Type C, which virtually is never associated with epidemics and is much rarer.

There are literally scores of upper respiratory viruses that cause symptons similar to those of influenza, and many intestinal viruses that their victims confuse with flu.

In face, had the Children's Hospital laboratory not isolated the flu virus in three children, staff doctors there would not have known that the disease they are starting to see in emergency room patients is probably flu.

The bug is not yet emptying classrooms and offices, and it is unlikely that it will this winter.

The peak of the flu season usually occurs just a few weeks from now, and according to the federal Center for Disease Control in Atlanta, flu has been reported in only 19 states so far this winter.

Additionally, Type B flu usually does not spread through a community as fast and as throughly as Type A does. "If you see it in Fairfax," said a CDC spokesman, "it may well stay in Fairfax. But if you had an A virus in Fairfax it would be all over the Washington area in no time."

While flu can lead to death in extreme cases, most people recover in less than a week, none the worse from a fever of up to about 103 degrees, chills, cough and aches and pains all over. In some cases, though, the victim will continue to feel weak for a few weeks.

There is a flu vaccine available, but it is recommended only for persons who face a high risk of serious complications, such as those with heart, upper respiratory or kidney diseases.

All that others can do is treat the symptoms rather than the disease itself. Doctors recommend asprin for the fever, plenty of fluids -- especially fruit juices -- cough medicine and bed rest.

If a high fever or cough persists, patients should contact their doctors about possible complications.

Although it is quite rare, there is one complication of Type B that parents should watch. Reye's syndrome, a condition that causes fatty buildup and degeneration of major organs, including the heart, sometimes accompanies Type B.If Reye's syndrome strikes, it is fatal in 42 percent of the cases.

If a child begins frequent, lengthy vomiting and runs a high fever at the end of a bout of flu, parents should notify the child's pediatrician immediately.