The U.S. is at war, definitionally if not legally, with a terrible hemorrhagic virus in Africa and an almost cartoonishly evil organization in the Middle East.

I believe, unlike some of my colleagues at The Week, that Islamic State of Iraq and Syria (ISIS) is a real threat, and that the U.S.-led air campaign against it is defensible. I also don't believe that the U.S. is at high risk of an Ebola epidemic, though a pandemic of a more communicable disease like influenza is a frightening possibility.

Nevertheless, of these two battles, the war on Ebola is the more important one, and the U.S. should focus its energies accordingly.

There are a lot of reasons for the primacy of the Ebola battle, ranging from the practical — the Ebola virus is simply a more dangerous enemy — to the moral and diplomatic.

First, the practical rationale. Ebola has run rampant in three poor West African nations — Liberia, Guinea, and Sierra Leone — whose health systems and governments have been weakened by years of civil war, ethnic conflict, and widespread poverty. It has spread to wealthier nations, including Nigeria, the U.S., and now Spain, all of which appear to have contained the virus.

But the numbers in West Africa are so alarmingly large, and the disease so unchecked, that further exports of the disease are virtually inevitable. Lena H. Sun, Brady Dennis, Lenny Bernstein, and Joel Achenbach lay out the daunting math at The Washington Post:

What happens next in the epidemic will be determined in part by mathematics. As of Friday, the WHO had reported 7,470 confirmed or likely cases, and 3,431 deaths in Guinea, Sierra Leone, and Liberia. Currently, each infected person is infecting about two more. To slow the spread of the disease and eventually stop it, officials must somehow reverse the math. Only when each Ebola patient infects, on average, fewer than one person will the outbreak begin to fade. [Washington Post]

The World Health Organization (WHO) says that at the current trajectory, there will be 20,000 Ebola cases by November; the U.S. Centers for Disease Control and Prevention (CDC) estimates that in a worst-case scenario, Ebola could infect 1.4 million people by late January 2015.

Faced with those numbers, which could plausibly lead to the social, economic, and political collapse of West Africa on top of the humanitarian horrors already unfolding, the U.S. is sending in the troops. On Sept. 16, President Obama committed 3,000 U.S. troops, additional personnel from the CDC and U.S. Agency for International Development (USAID), and up to $1 billion to fight ISIS, mostly in Liberia. No other country has come close to matching that.

So after the ethical imperative to stop the disease from reaching apocalyptic levels (see: 1.4 million infections), this is the next reason the Ebola fight is important for the U.S.: America has the moral high ground for the first time in a long while. It's not just the money and troops, either. The medical nonprofits Doctors Without Borders — which specifically requested military aid in this crisis, for the first time in its long history — and North Carolina–based Samaritan's Purse were the first outsiders to sound the alarm about the Ebola outbreak, but the CDC saw the danger long before the WHO.

With ISIS, the U.S. has the support of plenty of countries, but the whole campaign is tainted by America's long, controversial — and, of course, recent — history in the Middle East. With Ebola, Obama felt perfectly comfortable shaming much of the world on Monday: "I'll be honest with you: Although we have seen interest on the part of the international community, we have not seen other countries step up. We've had some small countries that are punching above their weight on this but we've got some large countries that aren't doing enough."

Sending ground troops to fight Ebola is a battle nobody can argue with (well, almost nobody). Lots of people hate ISIS, but Ebola is an enemy everybody is against. And there's another important difference: While Ebola has no proven cure and respects no borders, it's also a killer we can defeat.

ISIS is an anachronism wrapped in shiny packaging. We've seen Islamist marauders before, and we'll see them again. As former intelligence officer John Schindler has repeatedly noted on Twitter, the U.S. faced a not dissimilar threat 200 years ago from the Barbary Pirates. Like ISIS, they were a nuisance with the capacity to disrupt and kill; and like ISIS, they weren't anywhere near the kind of existential threat that Nazi Germany or Soviet Russia posed to the United States — a point Ryan Cooper made pretty bluntly last week. (After years of battle, the U.S. eventually beat the Barbary marauders.) If the U.S. degrades and destroys ISIS, another group like it will pop up somewhere. You can't bomb an ideology or tactic into submission.

But Ebola is an enemy that, while invisible and mobile, we will likely cure or kill off with a mass-producible vaccine. If we don't focus on Ebola, the virus may cause deadly outbreaks in the U.S. or, more likely, less developed parts of the world.

That's not to say that winning will be pleasant or easy. "The people in charge of stopping the Ebola epidemic will have to do something that they have not been able to accomplish," write Sun, Dennis, Bernstein, and Achenbach in The Washington Post: "They must be even more aggressive, more ruthless, and more persistent than the virus — a mindless and implacable force carrying out its own genetic instructions."

So let's be aggressive, ruthless, and persistent in this battle. And as long as our coalition holds together in Syria and Iraq, we can keep on bombing ISIS on the side.