The World Health Organisation routinely spends about $200m per year on travel expenses, figures show - more than what it doles out to fight Aids, tuberculosis and malaria combined.
Last year, the United Nations' health agency spent about $71m on Aids and hepatitis, with €61m going towards battling malaria, data acquired by the Associated Press found.
To slow the spread of tuberculosis, the WHO invested $59m. Some health programmes do receive exceptional funding - with about $450m being spent trying to wipe out polio every year.
The WHO declined to say if it paid for director-general Margaret Chan's stay earlier this month at the 1,000-dollars-per-night Palm Camayenne hotel in Conakry, Guinea, but noted that host countries sometimes pick up the tab for her hotel stays.
At a time when the cash-strapped health agency is pleading for more money to fund its responses to health crises worldwide, it has struggled to control its travel costs.
Senior officials have complained internally that UN staff break new rules introduced to try to curb its travel spending, booking business class air tickets and five-star hotels.
Nick Jeffreys, the WHO's director of finance, said in September 2015 during an in-house seminar on accountability: "We don't trust people to do the right thing when it comes to travel."
Despite the WHO's numerous travel regulations, Mr Jeffreys said staffers "can sometimes manipulate, a little bit, their travel".
The agency could not be sure people on its payroll always booked the cheapest fares or that their travel was even warranted, he said, adding: "People don't always know what the right thing to do is."
Ian Smith, executive director of Dr Chan's office, said the chair of the WHO's audit committee said the agency often did little to stop misbehaviour.
He said: "We, as an organisation, sometimes function as if rules are there to be broken and that exceptions are the rule, rather than the norm."
Earlier that year, a memo was sent to Dr Chan and other top leaders, reporting that compliance with rules requiring travel to be booked in advance was "very low".
The document also pointed out that the WHO was under pressure from its member countries to save money.
The UN health agency said "the nature of WHO's work often requires WHO staff to travel" and that costs were reduced 14% last year compared to the previous year - although that year's total had been exceptionally high due to the 2014 Ebola outbreak in West Africa.
An internal analysis in March, obtained by the AP, found that only two out of seven departments at the WHO's Geneva headquarters met their budget targets and concluded that the compliance rate for booking travel in advance was between 28% and 59%.
Since 2013, the WHO has paid out $803m for travel. The WHO's approximately $2bn annual budget is drawn from the taxpayer-funded contributions of its 194 member countries, with the US being the largest contributor.
After he was elected, US president Donald Trump tweeted : "The UN has such great potential," but had become "just a club for people to get together, talk, and have a good time. So sad!"
The United Nations has such great potential but right now it is just a club for people to get together, talk and have a good time. So sad!— Donald J. Trump (@realDonaldTrump) December 26, 2016
The WHO said in a statement that nearly 60% of its travel costs were spent on sending outside experts to countries and for representatives from member countries to attend their meetings.
During the Ebola disaster in West Africa, the WHO's travel costs rose to $234m. Although experts said the WHO help was critical, some questioned whether the agency could not have shaved its costs so that more funds went to West Africa.
Dr Bruce Aylward, who directed the WHO's outbreak response, racked up nearly $400,000 in travel expenses during the Ebola crisis, sometimes flying by helicopter to visit clinics instead of travelling over muddy roads, according to internal trip reports he filed.
Dr Chan spent more than $370,000 in travel that year, as documented in a confidential analysis of WHO expenses which identified the agency's top 50 spenders. Dr Aylward and Dr Chan were first and second on this list.
Three sources said that Dr Chan often flew first class.
The WHO said its travel policy, until February, "included the possibility for the (director-general) to fly first class". It said Dr Chan flew business class, and asked for the policy to be changed to eliminate the first-class option.