When Mark Zuckerberg of Facebook and his wife, Priscilla Chan, donated $25 million last week to support the treatment of Ebola victims and their families, they gave the gift to the foundation that funnels private donations to the Centers for Disease Control, not one of the nonprofit groups that Americans typically shower with money during a humanitarian crisis.

Compared to the rush to donate after major disasters of the last decade or so, charitable giving to address the Ebola tragedy is almost nonexistent, and the relief agencies that typically seek donations after a catastrophe are mostly silent. “Have you had any email solicitations?” asked Patrick M. Rooney, associate dean at the Indiana University Lilly Family School of Philanthropy. “If there had been an earthquake or tsunami, my question would be who had solicited you and how many times? Americans aren’t giving because they haven’t really been asked.”

Ever since terrorists took down the World Trade Center in 2001, Americans have generously supported the organizations that swing into action after earthquakes, floods, cyclones, mudslides and other disasters. Propelled by the Internet and cellphones, which make giving as easy as clicking a button, Americans donated billions of dollars to help victims of the 2004 tsunami that devastated countries around the Indian Ocean, the 2011 tsunami and nuclear disaster in Japan, and the 2010 Haitian earthquake, among other calamities.

But the Ebola crisis is different, charity officials and experts say, though it is hard to say exactly why. Perhaps it lacks the visual drama of a natural disaster. Or it is harder for people to understand what their money can do to fight a disease with such a high mortality rate and no sure treatment. It is not even clear that providing food, housing and protective equipment will have any impact — or how those things will get where they are most needed.