Tom began acting up when he was 3 years old. He would refuse to go to bed and insist on having the television on all night. His mother, worried that Tom might have ADHD, took him to the family physician, who referred him to a specialist at the Child and Adolescent Mental Health Service in London for an assessment. Psychologists went to nursery school to observe the little boy and grew perplexed. Tom played quietly with other children and attended carefully to lesson tasks. Nursery teachers said they considered him developmentally advanced for his age. "It was clear this boy was a very different child at nursery than he was at home," says Sheila Redfern, a child psychologist at the service to which Tom was referred. What accounted for this discrepancy?

Redfern began to see Tom along with his mother. "It was clear she was depressed," says Redfern. As they talked, it became clear that she hadn't wanted to become pregnant with Tom. She already had several older children and was struggling to cope as a single parent. She had almost deliberately been neglecting him, and in turn Tom was provoking her to get attention. Mom is now receiving counseling, and Tom is already becoming less belligerent at home. Although Tom's mother had firmly believed Tom had some kind of diagnosable disorder, she's slowly coming to understand that the crux of her son's problems lies instead in the family.

We tend to think of health as a private blessing, sickness as a solitary curse, but the truth isn't nearly so simple—either for kids or for adults. "The myth is that health is all about individual choices and individual treatments," says Dr. Robert Ferrer of the University of Texas Health Sciences Center in San Antonio. "But we're embedded in families and communities, and they have a big effect on our options." Families affect not only the stresses we encounter but the genes we carry and the environments we live in. And though science has yet to disentangle these influences, it has amply documented their impact. In a newly published analysis of health records from across the United States, Ferrer calculates that family forces may explain up to a quarter of the variation in individual health. And as the articles in this Special Report make clear, that finding is more than a curiosity. The "family effect" has profound implications for patients, for physicians and even for policymakers.

Some of the best-known family influences are genetic ones. As Harvard's Dr. Howard LeWine writes in this issue, your chances of developing heart disease or breast cancer double if one of your parents or siblings is affected. The risk is even higher for people with two stricken family members. But family history is not written in DNA alone. Household income is an equally strong health predictor, and not just because wealth buys more medicine. "It has little to do with money per se," says Sir Michael Marmot, a British epidemiologist who has spent a career chronicling what he calls the "status syndrome." "More wealth means more autonomy, less stress and more opportunities for social participation." No one knows exactly how those factors get translated into lower rates of AIDS, obesity and heart disease, but the effects can be dramatic. Life expectancy is some 20 years greater in suburban Montgomery County, Maryland, than in downtown Washington, D.C. And recent studies suggest that people raised in poor families have less resistance to infection as adults—even if their circumstances improve.

Status aside, people who live under the same roof often experience the same health problems, even if they're not genetic relatives. When researchers at Nottingham University analyzed the health records of 8,400 married couples, they found that the spouses of people who suffered from asthma, depression, ulcers or hypertension were at sharply increased risk of developing the same conditions. In those instances, the couples' common problems probably resulted from their shared diets, lifestyles and environments. But researchers have also documented numerous instances in which one family member's health problems affect the well-being of —others. "A traumatic event within the family can have consequences for everyone," says Dr. Nicholas Christakis of Harvard Medical School. "The collateral effects can include real, biological illness—even though the causes are entirely social."

We're all more prone to illness and death when our family members are in trouble. Surviving spouses die at twice the normal rate during the first year of bereavement, as they become more isolated and less motivated to take care of themselves—and the risk of suicide increases 22-fold for people whose spouses take their own lives. The loss of a child has a similar impact, roughly doubling a woman's odds of being hospitalized for depression even after five years have passed. And family crises can have a range of consequences for kids. Girls who lose a parent through death or separation are prone to depression and anxiety disorders as adults. And kids with depressed mothers exhibit more than their share of psychiatric and behavioral problems as teenagers—an outcome that researchers have tied directly to the poor parenting they receive.

The outcomes aren't always so dire. Living with a disabled or chronically sick child may not kill anyone, but the experience can change parents' and siblings' lives indelibly. When a son or daughter is diagnosed with a condition that can last a lifetime—juvenile diabetes, cancer, Down syndrome, autism—parents face an array of practical and emotional issues. Can the child attend a mainstream school? What will his future be like? Can the family afford his next treatment? Many parents put their jobs and lives on hold to become full-time caregivers, shuttling the child back and forth to hospital appointments and therapy sessions. Some families go bankrupt, and many more feel isolated and helpless. The strain on a marriage can be overwhelming, says Evan Imber-Black, director of the Center for Families and Health at the New York-based Ackerman Institute for the Family. The mother often becomes the child's full-time companion and advocate, leaving the husband to wonder, "What happened to our relationship?"

Siblings get eclipsed, too. "They feel forgotten and insignificant," says Melanie Goldish, the director of SuperSibs!, a support group for the brothers and sisters of kids with cancer. "There's a sense of abandonment or isolation." And though they're the first to acknowledge that a sick brother or sister has overwhelming needs, they end up aching for attention. Goldish recalls a woman named Stacy who spent her teens watching her brother fight a losing battle with cancer. Stacy told Goldish her name in school was "Greg's sister," and her nickname was "the well one." Anyone would resent that—but for kids with sick siblings, the resentment is often compounded by guilt. Frankie Romano was 10 when he and his little brother, Michael, then 4, spent an afternoon at the park with their grandfather. Frankie remembers tackling the little one and hurting his stomach. Several months later Michael was diagnosed with an abdominal neuroblastoma. "I was really broken down," says Frankie, now 17. "I thought that was my punishment." Frankie's mother, Sharon, confirms his account. "I remember the nurse holding him," she says. "He was crying, saying, 'Did we fight too much? Did we wrestle too hard? Did I hurt him?' "

We may never fully grasp all the forces that families exert on our health. But there are practical lessons in what we know already. An obvious one for anyone confronting a loved one's illness is to look beyond the patient. "Parents need to be as aggressive in seeking help for themselves and other family members as they are in focusing on the affected child," says Dr. Ed Clark, chair of pediatrics at the University of Utah. Some hospitals now sponsor support groups for family members, and organizations like the Seattle-based Sibling Support Project offer workshops where kids with special-needs siblings can learn strategies for coping. They have lifelong responsibilities, says founder Don Meyer, "and a lifelong, ever-changing need for information."

Thinking beyond the patient is especially important as the end of life approaches. Most people define a good death as one that doesn't burden the rest of the family, says Harvard's Christakis. Yet a third of U.S. families lose all or most of their savings the first time somebody dies. Hospice care can reduce the cost substantially—and studies suggest it can even help ward off depression and death in surviving spouses. In a study involving 35,000 couples, Christakis found that bereaved widows and widowers were less likely to die within 18 months if their partners had received high-quality palliative care. "Anything that happens to me has consequences for the people around me," he says. "To gauge the true value of my treatment, you have to consider its effect on the family and the community."

In other words, sickness and misfortune aren't the only family forces that merit attention. Just as individual problems can have repercussions for others, an intervention that improves one life can have big collateral benefits. Consider what happened in Elmira, New York, when University of Colorado psychologist David Olds dispatched nurses to drop in every few weeks on 400 high-risk, low-income teenage girls during their first pregnancy and their first two years as mothers. The nurses counseled the moms on nutrition, and helped them cut back on drugs, alcohol and cigarettes. They also offered basic instruction in life skills and infant care, and worked to keep fathers and grandparents involved. Olds and his colleagues figured the program would help kids as well as mothers, but the results surpassed their highest hopes. The program sharply reduced abuse, neglect and injuries in the participating families—and the benefits were still accruing when researchers followed up 15 years later. As teenagers, the kids had 81 percent fewer criminal convictions than peers from similar backgrounds, not to mention fewer sex partners, less substance abuse and fewer instances of running away from home.

Programs based on the Elmira experience are now reaching 20,000 high-risk mothers in 20 U.S. states through a group called the Nurse-Family Partnership. That's a tiny fraction of the number who could benefit; there are 26,000 in New York City alone, says director Clay Yeager. But the effort stands as a testament to what health care can accomplish by focusing on families. "It can change whole life trajectories," says Dr. Jeffrey Kaczorowski of the University of Rochester. That's a goal to aspire to.

With Claudia Kalb, Anne Underwood, Karen Springen and Tara Pepper