Ebola has infected just a handful of Americans, but millions of us are already sick with anxiety.

More than half of U.S. adults worry that there will be a large-scale Ebola outbreak across the next year, according to a new Harvard poll conducted last week and released on Tuesday. Most of them are nervous that they’ll get sick with Ebola, or someone in their family will.

Those numbers have climbed from an earlier Harvard poll, which found that about 40% of American adults this summer were worried about an Ebola outbreak. And this week’s anxiety-inducing news that two Dallas nurses had been infected with Ebola — with one Ebola-stricken nurse even flying a commercial airline between Cleveland and Dallas — is bound to drive fears even higher.

Is there cause for concern? Sure; Ebola’s one of the most dangerous diseases on the planet. The current outbreak has killed more than 4,000 people in West Africa, with a staggering 50% mortality rate.

But at the same time, scientists and public health officials have repeatedly urged Americans not to panic about Ebola.

Also See: Yes, Ebola Is Scary. But It’s Also Beatable. Here’s Why.

One of the most common questions I’ve gotten the past few weeks concerns a seeming paradox:

If Ebola’s so hard to catch, why are there so many photos of health care staff wearing full-body protective gear?

Or a different way to think about it, often asked by skeptics:

If Ebola’s so much of a threat, why aren’t more Americans getting sick?

It’s a visible inconsistency that’s threatened to muddle public health messages about Ebola. For example, the White House has shifted into Ebola “crisis mode,” Reuters reported on Tuesday, even while stressing that other Americans should remain calm.

Ebola in Four Charts
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WHO / Chart CC BY 4.0: JV Chamary / Source: http://onforb.es/1sCVxE1 Ebola cases in West Africa
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Ebola cases in West Africa

The total number of cases is rising at an exponential rate. As of 14 September, the doubling time is 16 days in Guinea, 24 days in Liberia and 30 days in Sierra Leone.

Here’s how to answer those questions, and understand the nuance behind why Ebola’s so scary — but not an imminent threat to most Americans.

With Enough Proximity, Ebola Can Be Very Contagious

The disease is “highly contagious” in the sense that an Ebola patient is a significant risk: As the patient’s symptoms worsen, the patient can begin projectile vomiting and experiencing explosive diarrhea — and those fluids have the capability to infect others.

The dried virus can live on doorknobs and countertops for several hours, too.

So the reason for the full-body HAZMAT suits? Individuals who end up in close proximity with Ebola patients are at risk for contact with those patients’ fluids. And those suits can end up soiled with blood, vomit, and other substances, adding another layer of danger when people try to take them off, too.

“When you have potentially soiled or contaminated gloves or masks or other things, to remove those without any risk of any contaminated material … [it’s] not easy to do right,” CDC director Dr. Tom Frieden said earlier this week.

Simply being breathed on doesn’t appear to infect anyone with Ebola, but droplets expelled by an Ebola patient — say, when the person sneezes — could theoretically transmit the disease.

(In an email, Forbes contributor David Kroll passed along this interesting post from virologist Ian Mackay, which explains that while Ebola may not be “airborne,” a person can still be in danger of being hit by “splatter” from an Ebola patient.)

Taken together, health care staff are at exceptionally high risk, especially because Ebola patients become most contagious at end-of-life. A health care worker who’s been around Ebola patients may, without thinking, rush to provide aid in a critical emergency, and later accidentally touch his or her own eyes, abrasions, or other openings on the skin.

You can see the danger to health care workers reflected in the current toll of Ebola infections and deaths. Doctors, nurses, and other health care staff represent about 6% of deaths in the current Ebola outbreak.