90-Second Read: Hantavirus takes a long time to show symptoms. Experts say more cases could appear

Because symptoms can take weeks to show up, additional cases could still emerge, complicating public health efforts to contact trace possible exposures and transmission chains. Of the 18 passengers who returned this week to the U.S., at least one person has tested positive for the Andes Hantavirus, which has killed three passengers who had been on the 147-person cruise that was diverted to the Canary Islands. On Tuesday the World Health Organization director announced that a total of 11 cases have been reported in the outbreak. The remaining two-a symptomatic person and their spouse who had no symptoms-were transferred to Atlanta for treatment and monitoring.

Based on the timeline of cases in the current outbreak reported to date, Pastula estimates that the incubation period has been two to three weeks, which is within the expected range. Although Hantaviruses are primarily spread through exposure to infected rodents or their feces or urine, the Andes virus is the only Hantavirus type known to spread human-to-human. Longer incubation periods can reduce pathogens' ability to spread quickly from person to person, which typically gives public health officials the upper hand in identifying contacts and monitoring them to control outbreaks. The Andes virus's comparatively long incubation time could also reflect how it attacks the body and initially evades immune detection.

HHS officials also said that risk of Andes virus to the general public is still low. But the situation is rapidly evolving and things could change. That's less clear with Andes virus, which can cause "widespread infection without developing an immune response [at first]." Research shows the Andes virus can quietly circulate in the bloodstream from the initial site of infection. Studies have shown that the virus can be present in the blood and other bodily fluids weeks before symptoms start.

Her work has appeared in Scholastic MATH, School Library Journal, IEEE Spectrum, Atlas Obscura and Smithsonian Magazine. Scientific American has served as an advocate for science and industry for 180 years, and right now may be the most critical moment in that two-century history. Young is associate editor for health and medicine at Scientific American.