Bill Gates says there are many lessons to be learned from the Ebola epidemic raging in West Africa. The biggest, he says, is that the world has far to go before it is prepared to combat an even-larger disease threat.

“The world as a whole doesn’t have the preparedness for epidemics, and we’ve had a few flu scares that got us to do some minor things, but not enough,” he said in an interview at the annual meeting of the American Society of Tropical Medicine and Hygiene here. “If this thing had been twice as transmissive, we’d be in a lot of trouble, and there are agents that have a real chance of coming on in the next several decades that are far more transmissive than this is. What’s to stop some form of SARS showing up?”

Or “SARS II,” he said—a contagious new disease that emerges in an impoverished country.

The billionaire philanthropist came to the conference mostly to speak about malaria—a disease the Bill & Melinda Gates Foundation says the world must eradicate and on which it is spending more than $200 million this year.

But the foundation is also contributing $50 million toward fighting Ebola, making it one of the largest donors to the response. It is spending money on efforts including experimental drugs for Ebola and having engineers devise a way to cool protective suits so medical staff in West African Ebola treatment units can wear them longer.

Ebola is a topic of discussion at this conference, both scientifically and because Louisiana officials last week asked people who had registered not to come if they have been to Guinea, Sierra Leone or Liberia or been in contact with an Ebola patient within the past 21 days.

Mr. Gates appeared moved after a 90-minute meeting at which two doctors who survived Ebola spoke. The epidemic is a key moment for global health, he said. “I think there’s a lot of lessons for future epidemics.”

Discussing the foundation’s reasons for getting involved, he said, “This disease, not only does it kill directly, it also shuts down health systems. If this thing had spread throughout West Africa, among other things you could throw out the window is the incredible progress we’ve made on polio.”

Luckily, that hasn’t happened, he said, even though experts from Nigeria’s polio program were diverted to help in the successful fight to stamp out that country’s own Ebola outbreak. Polio eradication is the foundation’s top health priority.

Most needed, Mr. Gates said, is a better system of global disease surveillance. While the Centers for Disease Control and Prevention and the Gates Foundation have invested in helping countries build capacity for monitoring for outbreaks, and while surveillance for new flu strains is well developed, far more is needed for existing and emerging diseases, he said.

“With new tools, doing broad disease surveillance that’s better than we have been doing is it a very doable thing,” he said. “You can do a pretty good job literally for hundreds of millions, not billions of dollars a year.”

“You have to gather samples on a systematic basis and send them to genetic analysis,” he said. “You have to take respiratory samples, diarrheal samples, on a regular basis and whenever you see elevations in death rates, then you have to have the capacity to oversample in those places.”

Primary health-care systems must also be strengthened, Mr. Gates said, noting that countries such as Ethiopia and Rwanda have shown that can be done at a modest cost.

And more money needs to go into researching and developing new tools such as drugs and vaccines to fight Ebola, he said.

The foundation is considering giving funds to Chimerix Inc. to help it boost production of brincidofovir, an antiviral drug that has shown potential against Ebola. It has also given funds to GlaxoSmithKline for work on an Ebola vaccine, he said, and is exploring other interventions.

He said clinical trials of experimental drugs must move forward quickly and can be conducted without formal randomized controlled clinical trials, using drugs that are known to be safe and for which there is evidence that they reduce the amount of virus in a patient.

“If you give it to 20 people and you see their viral load drop in a way that you’re just not seeing with other people, that is powerful, powerful evidence,” he said.

“There are ways of getting things out there without doing placebo arms and having big delays,” he said.

Mr. Gates praised the Ebola response now under way, particularly the role that the U.S. government has played in sending the military to build hospitals and ferry supplies, epidemiologists to track the epidemic, and more. “I give us a pretty high grade for what’s been done,” he said.

“Everyone looking back at this epidemic will find things they could have done better,” he said. “The fundamental lesson shouldn’t be about who did what quarantine when. That’s rounding error stuff compared to true preparedness for a seriously transmissive epidemic.”