Children's normal affectionate beheviour is part of the reason. Toy sharing, hugging and kissing are all quick ways to pass germs, but shouldn't be discouraged. What should be encouraged is better hygiene, especially handwashing. Respiratory infections are the most common childhood illnesses. They're in saliva and nose secretions.
Respiratory germs float through the air and are inhaled or settle in the eyes, nose or mouth.
Colds and flu are examples of infections caused by germs that spread this way. Ear infections are also caused by repiratory tract germs. Strep throat germs are spread when you sneeze or cough. Runny noses also spread respiratory infections, especially if caregivers don't wash their hands after wiping noses. Often, you don't feel sick for the first few days of having a respiratory infection.