According to the British Medical Journal, (February 17, 2001; 322: 390-395) it is a good thing when a baby gets a cold. Colds and minor infections seem to help the immune system to develop and help to prevent asthma and allergies latter in life. The finding supports a theory that an immune system that has been geared up to fight infection is less likely to overreact to innocuous substances.
While repeated mild infections seemed to help prevent asthma and allergies, recurrent serious infections were another matter. Serious infections of the lower respiratory tract, like pneumonia or the flu, seemed to increase asthma risk. The researchers point out that the children who have a tendency to get asthma may be more prone to these more serious infections.
Other research has found that children living on farms or with pets are less likely to get asthma or allergies. All of this research supports the idea that environments that are too sterile may not allow the immune systems to develop properly and causing them to overreact to harmless substances.