In one study, he looked at the feelings of revulsion people have to the smell of sickness. He took 8 participants and injected half of them with a compound that activated their immune system. The other half was given a placebo. Olsen then collected the scents of these 8 participants and had a group of 40 people rate the smells. The scents of the sick people were rated as less pleasant than the scents of the healthy people. This suggests that illness can be detected through smell.