The lung microbiota consists of microbes originating from the upper respiratory tract that migrate deeper into the respiratory system. These microorganisms are controlled by immunological responses as well as competition from preexisting microbes. The homeostasis involved in maintaining the lung microbiota is important in preventing and mitigating respiratory diseases. For example, cystic fibrosis and asthma are both respiratory diseases where there is an increase in the diversity or number of bacteria in the lower respiratory tract.