Pneumonia is usually caused by infection with viruses, bacteria or other germs. Common organisms that cause pneumonia include Respiratory Syncytical Virus, Influenza virus, Streptococcus pneumoniae and Mycoplasma sp. Most children who develop pneumonia are previously healthy with normal immune system. Pre-existing illnesses (e.g. chronic lung disease, weak immune system etc.) increase a child’s risk of contracting pneumonia.