Ability to repay is one of the top credit factors. Having the existing debt paid off improves ability to repay any future debt. If anything, the prospect of Bill Gates going around randomly paying debts makes issuing debt more appealing.

The article isn't clear, but I'd guess from the small outbreak in 2016 after being Polio-free in 2014/15 that the concern is that polio still isn't completely eliminated in Nigeria. Presumably it will need ongoing monitoring for several more years to ensure it is completely gone. Having to pay back the debt could jeopardise that ongong effort. In which case, this seems a good investment.

A 3y interval without cases in any member countries is what's required for the WHO to call a region "polio-free". [1]

[1] - http://www.searo.who.int/immunization/topics/polio/eradicati...

Interesting perspective, haven't thought of it this way before. It does still provide some benefit to the country though, they can use that money for something else instead of having to pay it back to Japan.

Hopefully this will be a small nudge aiding the general development of the country, which could eventually let it earn an upgrade to its credit rating.