What causes a cold or flu? There are so many types of viruses, how can you tell which ones are causing these infections? Let’s get into more details in this article.
Which viruses cause the common cold?
Over two hundred different viruses can cause the common cold, and rhinoviruses are the most common culprits. Other viruses that can cause common cold include respiratory syncytial virus (RSV), human parainfluenza viruses (HPIV), adenovirus, common human coronaviruses (not to be confused with SARS-Cov-2), and human metapneumovirus.
Rhinoviruses are responsible for up to 35% of cases of common cold. These viruses may contribute to roughly half of asthma flare-ups and they also have been linked with sinus and ear infections. Researchers learned a lot about rhinoviruses over the years- including the genome of over 130 human rhinoviruses, which can be broadly classified in 4 different species. They also have many other details about how these evolved and their special characteristics. For example, multiple rhinoviruses can infect humans at the same time. Scientists are optimistic that special antiviral medication and perhaps vaccines will be available in the future for rhinoviruses.
Respiratory syncytial virus (RSV) and Human metapneumovirus (HMPV). RSV are responsible for common colds, but can also lead to infections of the lungs (pneumonia) and ear infections. Most people get infected with RSV by age 2, and later on get infected again during childhood and adult years. In most cases, RSV causes mild symptoms of a common cold, and in children may cause a low grade fever as well. However, RSV can cause severe symptoms and complications in infants, premature babies and elderly with chronic immune conditions with weakened immune systems. Children with severe forms of RSV are at higher risk to develop asthma later on in life. There is a special drug called palivizumab which can be given in children younger than 2 years of age who are at high risk to develop complications from RSV infections. Discovered in 2001, Human metapneumovirus (HMPV) belongs to the same family of viruses like RSV- the paramyxovirus family. The typical symptoms caused by these viruses include cough, fever, shortness of breath and nasal congestion. In addition to symptoms of common cold, HMPV infections can also complicate with bronchitis or pneumonia.
Human parainfluenza viruses (HPIV)In the US, HPIV infections are more common in the spring, summer, and fall. In addition to causing cold like symptoms, these virus can cause croup, which is the infection of the vocal cords, trachea and bronchi, bronchitis and bronchiolitis (which are infections of the air passages), as well as infections of the lungs- or pneumonia. In other words, human parainfluenza viruses can affect both the upper and the lower respiratory tract. These infections are common in infants, young children, and eldery, especially those with weak immune systems. Human parainfluenza viruses (HPIVs) commonly cause upper and lower respiratory tract infections. There are four subtypes of HPIVs, HPIV-1 and HPIV-2 infections manifest as common cold or croup, while HPIV-3 infections can lead to bronchitis, bronchiolitis and pneumonia. HPIV-4 is less researched but is known to cause respiratory illnesses that can be severe at times.
Adenoviruses can cause cold but can also mimic flu, they circulate all year around, and can affect people at all ages. So far, researchers identified around 60 types of adenoviruses. When they infect the respiratory tract, fever and a sore throat are the most common symptoms. These viruses can also cause acute bronchitis, pneumonia, as well as conjunctivitis. Sometimes they can also affect the digestive tract causing diarrhea, nausea, vomiting, and abdominal pain. In rare cases, they can cause bladder infections.
Coronaviruses. Common human coronaviruses, including several types like 229E, NL63, OC43, and HKU1 had been associated with cold-like symptoms. In the US, they circulate all year around, although they tend to cause more infections in the fall or winter. Runny nose, sore throat, cough, fatigue and fever are common symptoms caused by these viruses. They tend to cause mild to moderate infections. Most people had been exposed to viruses from this group at some point during their lives.
Which viruses cause the flu?
There are three common types of influenza viruses: A, B, C. The A and B types are responsible for the annual flu epidemics and have been in-depth researched by scientists. The research is ongoing, as all these viruses responsible for flu mutate over time. Influenza viruses affect roughly 1 in 5 people and cause body aches, cough, high fevers, and other symptoms. Type C usually causes milder symptoms. Type A influenza virus can spread from animals to humans, as it happened with avian flu and swine flu. Type A influenza then spreads from person to person or from touching surfaces infected by others -by sneezing for example. Type B influenza usually infects humans only. In addition to flu symptoms, these viruses can lead to pneumonia, ear infections, sinus infections, muscle inflammation, and heart inflammation. Since 2017, the CDC has been assessing the severity of flu by age groups. At this time (2021-2022 season), the flu activity is assessed as being low, but this can change. For updates check this link. The flu shot is changed every year, based on the strains that are more likely to circulate during the cold season. The current flu shots are quadrivalent, which means they protect against four strains: two influenza A viruses and two influenza B viruses.