Pop quiz: Which of the above photos depicts a healthy person, and which depicts a sick person? Can you spot the subtle differences? (The correct answer is listed at the bottom of this article.)

Scientists have determined that humans have a remarkable ability to detect when someone is sick a mere two hours after the person is infected, and they can do this just by looking at a person's face, reports MedicalXpress.

For the study, 16 healthy Caucasian volunteers were given an injection of sterile molecules derived from bacteria. Although these injections were sterile, they nevertheless induced an immune response that made the subjects experience flu-like symptoms for a short amount of time. Their pictures were taken two hours after the shot to document their appearance.

Those same participants were asked to return on another occasion to receive another shot, but this time they were unknowingly injected with a placebo. Photos were again taken two hours later.

Both sets of photos were then shown to a different group of people, who were asked to rate the health of the subjects depicted in them.

"The raters could correctly discriminate 13 out of 16 individuals (81 percent) as being sick," said the study published in the journal Proceedings of the Royal Society B.

Although the sample size is relatively small, that's a detection rate that is significantly higher than chance. It's one thing to be able to detect whether a person is sick when we can observe their behavior-- for instance, people cough, sneeze, sniffle and show signs of fatigue. But it's another thing to be able to detect sickness from a neutral photograph alone.

Some of the signs of illness that observers pointed out included paler lips and skin, a more swollen face, droopier mouth corners and eyelids, redder eyes, and duller, patchier skin.

Study co-author John Axelsson of Stockholm University offered an explanation for the remarkable findings: "An ability to detect sick people would allow people to avoid being close to sick people, and hence minimize the risk of becoming sick if the person is a carrier of contagious disease."

The next step will be to expand the research, to see whether the results hold over a larger and more diverse sample. It will also be important to test whether different types of infection can be detected at similar rates. Eventually, computer software might be able to be developed from this research that can help doctors to identify sick patients.

Pop quiz answer: The person on the left (a) is sick, and the person on the right (b) is the healthy one.

Can you tell whether a person is sick just by looking at a photo? Science says yes
In one of these photos, the person is sick. Can you guess which is which?