Causes of Respiratory Infections

Viral infections cause most upper and lower respiratory tract infections. However, some bacterial infections also cause respiratory illnesses. Regardless of the cause, viral and bacterial infections have the same method of transmission: through direct contact. For example, you breathe in infected airborne particles from a cough or a sneeze. Or touch a contaminated surface. People with pre-existing health conditions, breathing problems, immune conditions, very young children, pregnant women, and the elderly have higher risks of infection and complications. 

Viral Infections

The Flu

Influenza viral infections cause the flu, a widespread and contagious respiratory infection that becomes an epidemic around the autumn and winter times. More often, this viral infection affects younger children. However, vulnerable populations, like those with pre-existing health conditions or older people, are more likely to have life-threatening complications, such as pneumonia and heart or lung conditions. 


Parainfluenza virus is another kind of viral infection that causes respiratory infections. These infections affect the lower respiratory tract, notably the lungs and lower airways. An infection is more likely to happen during the warmer months, like in the spring, summer, and fall. Reinfections can happen at any time because there is no way to prevent this viral infection. However, the good news is that symptoms tend to be mild in most people. 

Human Rhinovirus

Rhinoviruses are responsible for causing most respiratory tract infections, especially in young children. Rhinovirus infections cause the common cold. They contribute to missed days from school and work and increased costs associated with managing symptoms. However, this viral infection can be life-threatening in people with respiratory conditions like asthma, COPD, and cystic fibrosis. Pre-existing health conditions also increase the risks of infections and complications.


The human adenovirus is another viral infection that causes respiratory infections. Unlike other viral infections, adenoviruses do not have a distinct infection pattern. However, they peak around the winter and early springtimes. Adenovirus outbreaks tend to happen in places with crowded living spaces, such as military barracks, college campuses, and nursing facilities. Most people experience mild symptoms. Severe infections are rare, but they can cause respiratory failure.

Human Metapneumovirus (HMPV)

The human metapneumovirus (HMPV) is not just any viral infection. It is the leading cause of upper respiratory infections in vulnerable groups, like young children, older adults, and people with pre-existing immune conditions. Most of the time, infections occur in children younger than five. But, reinfection can happen later in life; hence, adult infections. HMPV infections cause respiratory symptoms in most people. However, in severe cases, an infection can result in uncontrolled asthma or COPD, or pneumonia. Similar to other viral infections, HMPV follows a seasonal pattern. It peaks between the late winter and early spring. 


Coronaviruses are a large family of viral infections that cause upper respiratory infections in animals and people. In people, coronavirus infections cause mild to severe respiratory infections and illnesses. The coronaviruses that affect people (SARS, MERS, and COVID-19) are highly transmissible and contagious. They have been responsible for global health epidemics.

Human Bocavirus

The human bocavirus is a relatively new viral infection that seems to affect young children. High amounts of the human bocavirus cause respiratory symptoms, like wheezing and airway inflammation. There’s also a relationship between infections and community-acquired pneumonia, middle ear infections, and worsening of asthma symptoms. Human bocavirus infections can happen at any time. However, they are likely to occur with other viral or bacterial infections.

Bacterial Infections

Streptococcus Pneumoniae

Streptococcus pneumonia (“S. pneumonia”) is a bacterial infection that causes different kinds of infections, like middle ear infections, community-acquired pneumonia, meningitis, and respiratory infections. S. pneumoniae bacterial infections happen in children and adults, though they occur more frequently in children during colder months. 

Haemophilus Influenzae

Haemophilus influenzae type b (Hib) is a bacteria that causes inflammation, infection, and diseases, such as pneumonia, lung infections, ear infections, airway disease, and joint inflammation, frequently in children five years or younger. The use of Hib vaccines significantly cut down the rate of Hib disease. It also decreased infection rates. However, Hib bacterial infection often causes bacterial meningitis and lower respiratory tract infections in children and adults.