Viruses, bacteria, and fungi are among the common causes of respiratory infection, with viruses leading the way as the most common. Bacteria are usually the next most common, and fungi generally cause respiratory infections less frequently. Sometimes, the initial cause of a respiratory infection is irritation to the respiratory tract that leaves the person vulnerable to infection. Most people suffer from minor respiratory infections from time to time and may develop serious infections only rarely. An individual with a depressed immune system is more likely to develop a serious and hard-to-treat infection than the average person, but anyone can fall victim to a serious respiratory illness.
In most places, viruses are top on the list of causes of respiratory infection. This is due to the fact that viruses that cause this type of infection are usually highly contagious and spread through droplets in the air as well as through contact. For example, a person could catch a virus by breathing in the same air space in which an infected person has just coughed or sneezed. One can also catch a virus by kissing an infected person or by touching a surface that has the virus on it and then touching his eyes, nose, or mouth.
Bacteria are also among the common causes of respiratory infection. Sometimes these infections are contracted in much the same way as viruses, but they can also develop in other ways. For example, if a person inhales a substance that contains infection-causing bacteria, he may become ill. A person also can develop a viral infection that becomes complicated by a bacterial infection — this is a particular concern for people who have weakened immune systems. For example, a person may develop the flu, but instead of getting better in a week or so, it progresses into a bacterial lung infection.
The common causes of respiratory infection include fungi as well, though more rarely. Sometimes a person inhales fungal spores and becomes ill as a result, but it is not always easy to identify the cause of the infection in this case. In other cases, a fungal infection may start in another part of the body and then spread to infect the respiratory system. Fungi can infect anyone, but those with weakened immune systems may prove the most vulnerable. For example, a person who has an immune system disease such as acquired immune deficiency syndrome (AIDS) may prove more likely to develop a fungal respiratory infection and have trouble fighting it than a person with a normal immune system.