African leaders have called for more funding for the UN's Global Fund, which fights Aids, tuberculosis and malaria.
Money from the Global Fund should give greater access to Aids drugs
For the first time, the fund is holding its board meeting in Africa, where the three deadly diseases are widespread.
African leaders are pressing for the two-year old fund to launch a fresh appeal to donors at the Arusha meeting, but the US is thought to be reluctant.
President Benjamin Mkapa of Tanzania, the host nation, said a refusal would send Africans "a wrong signal".
"Frankly, meeting for the first time in Africa, and not deciding to launch a new funding round, will be difficult for Africans to understand," said President Mkapa.
"It is my hope that the meeting will give Africa reason for more hope, not despair, and reason to believe that ours is still a just world that will not shut its eyes and ears," he said.
His appeal has the backing of three other African presidents at the meeting: President Paul Kagame of Rwanda, Mwai Kibaki of Kenya and Yoweri Museveni of Uganda.
President Kagame said combating deadly diseases needed "total commitment by the donors".
So far over $3bn (£1.62bn) has been committed to the Global Fund, which was set up in 2002 to combat Aids, tuberculosis and malaria. Nearly two-thirds of this money has gone to Africa.
The Global Fund is suffering from a shortfall, which the US has blamed on other donors not meeting their pledges. It is reluctant to endorse a further round of fund-raising.
"The US is the single largest contributor since the Fund was launched...This is war we have to win collectively," said US health secretary Tommy Thompson.
African campaigners argue that the world's deadliest diseases threaten the continent with a humanitarian and economic disaster.
Nearly half of those infected with HIV worldwide live in sub-Saharan Africa and one person dies every five minutes from malaria in Tanzania alone.
Tanzania's Commission for Aids says it hopes to receive almost $400 million (£216 million) from the fund over the next five years.
Its chairman said: "You can never have enough money in the fight against Aids because we are talking about advocacy, we are talking about treatment which is a very expensive affair.
"You'll see that $400 million is literally a drop in the bucket, especially if you are talking about lifelong treatment for some of the patients."
The UK's Secretary of State for International Development, Hilary Benn, will also be present in Arusha.
The UN has urged Prime Minister Tony Blair to use Britain's upcoming presidency of the G8 group of rich nations to lobby for more funds for health care in Africa.
President Mkapa is to open a factory making insecticide-treated mosquito nets on Wednesday, which has been set up with money from the Global Fund.
The fund is already starting to make a difference to those living with HIV and Aids in developing countries by giving more people access, or at least some hope of access, to anti-retroviral drugs.