Cases of meningococcal meningitis, a dangerous infection of the covering of the brain and spinal cord, have increased dramatically in all regions of the country in the first months of 1981.

Hardest hit have been Houston and Miami, where outbreaks of the illness have occurred. The disease also appears to be on the rise in Virginia, which reported 21 cases of meningitis of other infections caused by meningococcus bacteria for the first two months of this year, compared with 11 last year. Maryland and the District of Columbia have experienced no increase in cases so far.

By the end of February, 781 cases had been reported nationwide to the federal Centers for Disease Control, a 76 percent increase over last year's figure of 445 for the same period. If the trend continues, more cases of the disease will occur this year than in any year since 1954.

Experts at the disease centers theorize that this year's severe influenza season may have contributed to the rise by lowering some people's resistance to the disease.

Dr. Loreen Herwaldt, an epidemiologist at the federal centers, said health officials are concerned by the increase, but that so far it does not compete with the major epidemics that occurred in this country in the 1920s, '30s and '40s.

If the present rate continues, there may be 4,500 cases of the disease this year, compared with about 18,000 in 1943. Outbreaks in the armed forces contributed to the total that year, but no one really knows why meningococcal meningitis has become so much less common in recent years. "People think back to that and it sort of frightens them," Herwaldt said.

Meningococcal meningitis is caused by bacteria that enter the body through the nose or throat and infect the meninges, the delicate covering of the spinal cord and brain. Although antibiotics can cure the majority of cases -- particularly if given promptly -- about 15 percent of the victims die; most of the rest recover completely, according to Herwaldt.

She said a related, rarer form of meningococcal infection, in which bacteria enter the bloodstream, is fatal in as many as 29 percent of cases, sometimes killing its victim within hours.

Young children are vulnerable to the illness, particularly toddlers who play in close contact with one another at nursery schools or day-care centers. This year, for the first time, five cases occurred in a single classroom of an elementary school in Houston. In the past, epidemics of the disease have occurred among military recruits living in crowded barracks, but recruits are now vaccinated against it when they enter the armed forces.

Meningitis begins with a headache, muscle aches, and a fever that Herwaldt said usually ranges between 101 and 104 degrees. Small children often become irritable, refuse to eat and suffer nausea or vomiting, or they may simply become abnormally sleepy. As the disease progresses, patients often are confused and hard to arouse. If the bacteria enters the bloodstream, it can produce a rash that looks like tiny bruises, and a patient's blood pressure may fall so precipitately that he passes out.

Herwaldt said health authorities are puzzled by the sudden rise in cases. One theory, supported by some studies in past years and now being tested in Texas, would implicate this year's relatively severe influenza season in triggering the increase.

It suggests that during a cold or flu episode, a victim's natural defenses against bacteria are weakened -- either because the cells lining the throat are damaged or production of chemical antibodies decreases. Meningococcus bacteria, which are often carried in the throat without causing any illness, may then get past the weakened barrier and produce an infection.

Changes in the bacteria may also be a contributing factor. Herwaldt said there are various strains of meningococcus bacteria, which differ in the makeup of their chemical capsule, and people can be resistant to one strain but not to another. Although a vaccine has been developed against types A and C, type B -- for which no vaccine exists -- acounts for the majority of cases in this country.

So far this year nine cases of the disease have occurred in Maryland and one in the District -- no more than expected for late winter. It is too soon to tell whether the rise in cases seen this year in Virginia will turn out to be significant, according to Dr. Harry C. Nottebart, director of the state bureau of communicable diseases.

Herwaldt said that since the bacteria are carried in the nose and throat, its spreading can occur when a carrier coughs or sneezes, and is commonest among family members or others in frequent close contact. Although the immunity conferred by the vaccine lasts only a year, the vaccine -- and a course of antibiotics -- are recommended for anyone who has been in close contact with a victim of the disease.