In 1870, a European exploring Africa accidently brings back an African plague such as ebola, which wipes out much of the population of Europe. A few isolated areas such as Andorra remain unaffected, since they're out of the way, unpopular, with low population.
Many people flee to the Americas, or to North Africa.
Few people with the disease are allowed on the ships so the pandemic doesn't spread much beyond Europe.
Universities in the Americas spend far more time and effort on medicine, studying what's going on in Europe, and medical sciences become a lot more advanced.
By the mid-20th century, cures for the African diseases have been found, along with cures for cancer and other diseases. In the meantime, health clubs, healthy eating and related ideas become a lot more widespread.
During this period, the European colonies in Africa become balkanized and independent. Their wealth increases due to the oil and mineral resources. Swaziland is one of the independent countries. Many of its prominent people had studied in the US, and adopted Western ways, so it was allowed to retain its autonomy.
People in the Americas perceive Europe as being a nasty, brutal place full of disease and war, so there's little inclination to go there, except for researchers needing samples for their work. Politicians are not pressured into providing aid to Europe, the aid is expected to go down a metaphorical drain.
On the other hand, Africa is perceived as being full of European colonists taming a formerly wild continent.
So there's more contact and trade between the US and African countries. Many African countries become "first-world" countries, just as prosperious as the US. European countries are still small and poor.
How's that?