According to a recent study, excess cleanliness and a lack of exposure to germs during early childhood may negatively affect the development of the immune system, which can result in acute lymphoblastic leukemia, the most common cancer among children under four. Drawing on more than 30 years of research, the study recently published in the scientific journal Nature Reviews Cancer showed that one in every twenty children is genetically predisposed to this illness, but they do not develop it if they have healthy immune systems, which depends on exposure to a wide variety of common microbes during the first year of life. Contact with other children, pets, and dust and dirt prepares the immune system to fight harmful germs and avoid genetic mutations after a bout of cold or flu. In addition, when immune systems develop poorly owing to a lack of exposure to germs, the likelihood of asthma, allergies, and autoimmune diseases such as Type 1 diabetes and multiple sclerosis may rise. According to the authors, these types of illnesses have been increasing and are more common in developed countries where children grow up in overly clean environments.