At the same time, the United Nation's once overwhelming role in administering international health assistance has shriveled, with potentially serious consequences, the studies warn.
Aid earmarked for health in developing countries doubled in the 1990s and then again from 2001 to 2007, climbing from 5.6 to 21.8 billion dollars (4.0 to 15.6 billion euros) over 17 years.
But the distribution of those extra dollars, euros and yen have disproportionately favoured nations whose health needs are not among the world's most urgent, the research shows.
"Three of the countries among the top 30 with the highest disease burden -- Burkina Faso, Niger and Mali -- are nowhere near the top 30 in terms of the money they receive," said Christopher Murray, head of Washington University's Institute for Health Metrics and Evaluation and main architect of one of the studies.
"The gap has become more pronounced over time" even as more money flowed into all of these countries, he told AFP by phone.
The first comprehensive analysis of global health aid from all sources suggests that money flows are often linked to historical or geo-political factors, not just levels of illness and premature death.
"In some cases, health aid seems to coincide with defence spending or drug interdiction efforts," said lead author Mirmala Ravishankar, a researcher at the same institute.
Columbia ranks 23 for disease burden, for example, but 16th for development assistance, much of which comes from the United States. Two other Latin American countries -- Nicaragua and Peru -- also receive a greater share of the health funds pie than conditions in each country would seem to dictate.
Iran, subject to UN and bi-lateral sanctions for its alleged support of terror and its push for nuclear technology, is 22nd on the disease burden list, but ranks only 135th for per capita health aid.
The study, published in the British medical journal The Lancet, also found that funding has shifted substantially from public to private channels.
In 1990, UN agencies and development aid banks accounted for more than half of assistance dollars for health. By 2007, that share had dropped to 21 percent.
Two private-public hybrids -- the Global Fund to Fight AIDS, Tuberculosis and Malaria, created in 2002, and the Global Alliance for Vaccines and Immunization (GAVI), set up in 2000 -- handled more than 12 percent of assistance aid by 2007.
The share of resources moving through NGOs nearly doubled from 13 to 25 percent over the same period.
Several new private donors entered the arena as well, notably the Bill and Melinda Gates Foundation, which disbursed more than nine billion health dollars from 1998 to 2007, four percent of the global total in that year.
"It is good that we have a more diverse set of actors and institutions in global health -- we would not have seen a growth of 22 billion dollars if it was all flowing through the UN system," said Murray.
"But there are certain core functions that only the United Nations -- especially the World Health Organisation (WHO) -- can fulfill," he said, pointing to its key role in managing the current outbreak of swine flu. "The risk is that these functions may get neglected and underfunded."
The second study, also published in The Lancet, shows that the rapid rise of so-called "global health initiatives" (GHIs) has -- despite gains in the fight against specific diseases -- further weakened many already sick health care systems.
"Countries may decrease their own spending on specific diseases and sometimes health in general," found the study, prepared by the WHO.
In an editorial, The Lancet editor Richard Horton said: "A great deal more needs to be done to strengthen and measure the contribution of GHIs to a country's overall health sector."
Only five cents out of every dollar channeled to international health assistance goes to system-wide health support such as the funding of new clinics, doctor training and prevention programmes.
The share-out of donor dollars does not align with the misery inflicted by different diseases either, the studies show.
HIV/AIDS accounts for at least 23 percent of health aid, while tuberculosis and malaria -- which together cause more deaths -- receive less than a third as much.