Respiratory infections can be particularly insidious in a hospital, because they don't require any kind of incision or catheter to infect patients. Just as with pneumonia, lots of common scenarios in a hospital, such as being in a coma, using a breathing tube or lying prone for a long time, can make a patient more susceptible to respiratory infections.
The population of patients in a hospital might carry with them several different strains of respiratory infections, such a Legionella or influenza. These infections can be transmitted through the air and can spread through an entire building via air conditioning systems and other ducts. Legionella, for instance, is actually a waterborne bacterial infection, but contaminated water can spread the disease through air with the use of humidifiers and mist machines [source: Legionella.org]. Other illnesses, like tuberculosis, can be transmitted by air droplets produced when someone coughs or sneezes. These droplets can float in the air for a long time and be transported throughout a building.
To reduce transmission of these diseases, hospitals have to carefully consider their heating and ventilation systems. Proper filtering and cleaning of ducts is important, along with maintaining pressure differentials to control the direction of airflow within the building. For example, a patient undergoing surgery is very vulnerable to infection, so the surgical suite should have higher air pressure to keep potentially contaminated outside air from coming in [source: Pyrek].