MONROVIA, Liberia -- A young woman named Princess, already a refugee from civil war, was diagnosed with HIV in 2008. She began treatment but stopped taking her medicine. After an AIDS-related infection brought her back to the hospital, a nurse wrote out the words “life” and“death” on a piece of paper and told her to choose one. Since that clarifying moment, she has taken her pills faithfully. As a result, her son Michael, now 18 months old, was born HIV-free.

An HIV-positive friend of Princess’ stayed on treatment for two years but also lapsed, convinced that her good health was a sign she had been cured. Her child, Emett, was born HIV-positive. Five months later, the mother’s immune system failed and she died. The father refused to accept a child with HIV, so Princess took in Emett to raise as her own. She also volunteers to visit other AIDS patients, making sure they keep up with their twice-daily medication.

Princess’ story demonstrates the lavish generosity often practiced by those with nothing. It also reveals the vulnerability of women and children in an impoverished, conflict-prone country.

Liberia is a nation on the mend but starting from scratch. During more than a decade of civil war, much of the country was run by roving bands of drug-charged youth who raped at will and looted schools and clinics down to the tiles and wiring. All power generation in Liberia ceased. The nation’s GDP fell by 90 percent.

In 2003, one of the main public health challenges was not malaria or tuberculosis but unburied corpses. Twelve of Liberia’s 15 counties lacked a single doctor. Following the peace agreement in 2006, hospitals and clinics have been gradually repaired, sometimes with Chinese help. But the setback was massive. As other African countries were reducing child mortality, Liberia’s regressed. More than one in 10 children die before their fifth birthday — a tithe to savage gods of war and disease.

The death of a child is an assault on the idea of moral order. If this is not a crime and a blasphemy, there are no such things. Yet 7.5 million children under 5 die each year, six million of them from preventable diseases.

In this case, what is unnatural is largely unnecessary. The causes of child mortality are not mysteries. Two of the largest killers of children are pneumonia caused by pneumococcal disease and diarrhea caused by rotavirus — conditions preventable by existing vaccines. When children sleep under malaria nets, child deaths can decline by 30 percent. With proper treatment, the transmission of AIDS from mother to child is almost always preventable. Providing birth attendants with simple suction devices can help a child draw his or her first breath. Nutrition supplements and clean water are capable of saving many young lives.

Combine these approaches in various configurations, and you get the results measured in Senegal, Kenya or Rwanda — reductions in child mortality that are running at more than 8 percent a year. These are some of the swiftest gains in the history of public health, but need to be duplicated in places such as Nigeria, Pakistan, India, Ethiopia and the Democratic Republic of the Congo.

The task is hardest in post-conflict countries, where the health infrastructure has been decimated. Liberia is simultaneously attempting to distribute drugs, staff up nurses and doctors, and reconstruct ruined facilities. Yet the entire national budget is about half that of the Washington, D.C., public school system. Liberia’s total per capita expenditure is $125 per year. This is the reason foreign assistance to Liberia is essential, particularly during the next decade of transition. The U.S. contribution here is about $200 million a year, mainly devoted to fighting AIDS and malaria, promoting economic and agricultural development and training security personnel — a per capita American expenditure of about 70 cents a year.

In the absence of an effective government, even this relatively small commitment might be wasted. But Liberia under President Ellen Johnson Sirleaf is a relatively hopeful place. It is a functioning democracy. It has its fiscal house in order, has settled its debt problem and has begun attracting foreign investment. The economy is growing and those in the Liberian diaspora have begun returning. This is progress, but from a shockingly low level. Liberia’s planning goal is to reach the economic development level of nearby Ghana — in 15 or 20 years.

In the meantime, in Liberia and other desperate places, American policy means life or death. Some may not like it, but will choose one.