That's why food loses much of its taste when you have a cold. In addition to the smells we are so aware of every day, humans may have an ability to sense chemical signals from each other without being aware of it. Other animals have this second system which enables them to detect sexual and social information via substances known as pheromones. A whiff of a pheromone from a male rat in its territory may spur another male to a quick attack. Such signals exchanged between males and females incite mating.
Animals detect pheromones via a special organ located in the septum that divides the nose.
However, pigs recognize at least one pheromone by using the same sensors in the top of the nose that humans use to smell food. This leads researchers to speculate that humans have the ability to detect pheromones and these chemicals, in turn, influence their behavior. The best evidence that this happens comes from experiments showing that the menstrual cycles of young women who live together change gradually and involuntarily until they are in synchrony.