Most parents of infants and preschoolers have no memory of summers when children were kept out of swimming pools for fear of catching polio. Indeed, a 1993 Gallup poll shows that nearly half of parents today are unaware that polio is a contagious disease. Nor do they know firsthand the terrors of breath-robbing whooping cough, the often fatal paralysis of tetanus or the sometimes fatal throat infection caused by diphtheria. Many do not know that measles can cause life-threatening encephalitis and mental retardation. Nearly all parents of young children today grew up when these serious infectious diseases were prevented by vaccines, which have saved more young lives than any public health measure since the purification of public water supplies. This lack of firsthand experience has led to a frightening complacency about protecting infants and toddlers against the dangerous diseases that can be prevented by vaccines. "A lot of people have this cavalier attitude -- 'Oh, it's just a childhood illness' -- but that doesn't mean it's without risk," says Dr. Tony Luberti, director of ambulatory pediatrics at St. Christopher's Hospital for Children in Philadelphia. While more than 95 percent of children are fully immunized by the time they enter school, a requirement in all 50 states, the protection of infants and toddlers is woefully inadequate. Only 56 percent of all American children under 2 get the immunizations recommended by public health officials and the American Academy of Pediatrics. According to Maryland's Department of Health and Mental Hygiene, roughly 57 percent of the 2-year-old children in Baltimore have had their recommended shots. Statewide, the figures are about 62 percent. Yet preschool children are most susceptible to these diseases. Although immunization rates are low among children from poor families, who depend on government-financed health care, a study among the employees of Johnson & Johnson, all of whom have good medical insurance, found that middle-class families are also often delinquent in immunizing infants and toddlers. As a result, the United States ranks 70th worldwide in preschool immunization rates. While the Clinton administration has zeroed in on vaccines' cost, which has risen tenfold in the last decade, as the reason so many youngsters are inadequately protected, experts cite other factors as equally or even more significant. Perhaps most important is the failure of parents to appreciate the potential seriousness of the diseases. In the mid-1950s, when the Salk polio vaccine first was first available, few parents needed prodding to present their children for the long-awaited shots. But the Gallup survey, reported last April on a poll of 1,000 parents of children under 5, found that 47 percent did not know that polio is contagious, 36 percent did not know measles could be fatal and 44 percent did not know hemophilus B influenza is the leading cause of potentially fatal childhood meningitis. In the coming years, the cost of some vaccines is expected to drop. If President Clinton has his way, vaccines will be provided free to all children whose parents cannot afford them. To encourage more vaccinations, however, access to the clinics providing immunizations will have to improve greatly, with more staff and extended hours, perhaps evenings, weekends and holidays. New vaccines and improved ways to administer old ones are also expected. Meanwhile, parents must assume the primary responsibility for getting their preschoolers fully immunized. "Even though a shot might not be 100 percent effective on an individual child, what we tend to rely on is its effect on the population as a whole," says Dr. Luberti of Children's Hospital. "You're not doing it just for your kid, but for the kid down the street, too." But parents who are counting on this so-called herd immunity are also seriously misled. As the measles epidemics of a few years ago demonstrated, even when three-fourths or more of children are immunized, the unprotected can still get sick and die. Some parents resist certain vaccinations because of the fear of side effects, particularly with the pertussis -- or whooping cough -- vaccine in the well-known DPT (diphtheria, pertussis and tetanus) shots given to infants. Rarely, an infant develops seizures, which can lead to permanent brain damage, following the pertussis vaccine that is in common use in this country, and publicity about this effect has prompted some parents to take their chances with whooping cough. A Virginia-based group called Dissatisfied Parents Together, which says it has more than 35,000 members, targets the pertussis vaccine as being particularly dangerous. Barbara Fisher, who founded the group, contends her son, Christian, now 15, suffered permanent learning disabilities as a result of side-effects stemming from his fourth dose of the diphtheria-pertussis-tetanus shot at the age of 2 1/2 . The pertussis shot, she says, is the "oldest and crudest" in use. Although physicians have reported more cases of pertussis than usual in Maryland -- 136 cases so far this year -- state health officials attribute the rise partially to improved diagnosis: The state's health department issued a pertussis alert to increase physicians' awareness of the disease's symptoms, says Karyn Williams, health services specialist in the Immunization Division of the state's health department. To be sure, costs are a serious impediment for many families. Most insurance policies do not cover these costs, which usually include a physician or clinic fee. The cost of the vaccines alone now average more than $230 in the private sector and more than $112 in the public sector for all the recommended shots. The charges include a congressionally mandated federal excise tax to cover the costs of vaccine-related injuries. Out-of-pocket immunization costs have driven some middle-class parents to rely on already overburdened public clinics at a time when clinic hours and staff are being curtailed. Inconvenient hours and seemingly endless waits can mean no immunizations for many children whose parents have inflexible work schedules and who must make long trips by public transportation to a clinic. Doctors also contribute to the problem by not taking advantage of every medical opportunity to check children's immunization status and by being overly conservative about giving vaccines to children with minor respiratory infections. Dr. Georges Peter, former chairman of the pediatrics academy's committee on infectious diseases, maintains that only a moderate or severe illness or allergic sensitivity to a vaccine component should preclude vaccination. And experts who have studied reactions to the pertussis vaccine say the only children who should not get it are those over 7, those receiving immunosuppressive therapy and those with the following risk factors: a history of convulsions, a neurological disorder like epilepsy, a current fever or a history of a serious DPT reaction (allergic reaction, very high fever, collapse or shock, hours-long persistent crying or unusually high-pitched HTC crying, convulsions or impaired consciousness). Physicians now recommend acetaminophen to prevent high fevers after DPT shots. Under the newest recommendations, children should be getting their shots at about 2, 4, 6 and 15 months, with some booster shots later. Included are the two newest vaccines, one for hepatitis B, which should be given in a shot at birth, another at 1 to 2 months and a third at 6 to 18 months, and four shots for hemophilus influenza (Hib), starting at 2 months. Then come the old standbys: DPT shots should be given in four shots, starting at 2 months; the oral polio vaccine should be given in three doses, starting at 2 months, and the combined mumps, measles and rubella vaccine (MMR shots) should be given at 15 months. The Hib vaccine has now been combined with DPT to reduce the number of shots. Why is this early start so important? Last year, according to data from the U.S. Public Health Service, more than 2,000 cases of measles, more than 2,000 cases of mumps and more than 3,000 cases of whooping cough afflicted American children; many of the victims were incompletely immunized toddlers.