Viruses are everywhere all around us. They are in the air we breathe, the water we drink, and the food we eat. Viruses are even part of our natural microbiome and play an important role in immunity. Some viruses are good for us, while others cause infections. In this article, we will discuss how viruses cause respiratory infections and when such an infection may require professional medical evaluation.

Introduction

Airborne infections make up the largest class of infectious diseases. They are transmitted through small droplets that are expelled from infected people by coughs, sneezes or any other way a person can exhale, or by inhalation of contaminated dust. When we breathe in, our respiratory system gets exposed to millions if not billions of microbes, including bacteria, viruses, and protozoa. You may think that nature is constantly trying to kill us, but most of these microbes are not only harmless to a (healthy) person, but they constitute a large portion of the microbiome that has shown throughout the years to play a role in many body functions. Some microbes can however cause harm. For this reason, we have an immune system that is constantly scouring the body for invaders. Since we are breathing air constantly, the primary entrance point for infections is the respiratory system. When microbes enter through the nose, most of them are either brushed off by nasal hairs or get stuck to nasal mucus, thus preventing infection. Some pathogenic microbes do get through this primary defence system but the moment they reach the mucosal lining of the nose, most of them get neutralised by the mucosal immune system. Microbes that dodge this second line of defence as well can invade the host’s mucosal cells and cause infection. Some people are more susceptible to airborne diseases than others: in young children and elderly people, as well as immunocompromised individuals, the immune system can have a harder time fighting off infections; smoke exposure like cigarette smoke or air pollution can damage the respiratory tract, impairing its ability to clear off pathogens.

Stages of a Respiratory Infection

Now, let us discuss the progression of a viral respiratory infection. Respiratory tract infections typically progress through several stages.¹

 These include:

• Incubation period
• Prodromal stage
• Acute stage
• Convalescent stage
• Recovery stage

Incubation Period

If a virus manages to get through all innate defence systems, it comes into contact with the mucosal epithelial cells. Viruses are masters of disguise. They have co-evolved with humans for thousands of years, adapting to our bodies. Most viruses contain proteins that are recognised by our cells as their own, and viruses use this to get into the host and multiply as a form of parasite. When viruses enter a cell, they can hijack the host’s complex genomic machinery to start creating copies of itself. While the virus is multiplying, at first the immune system doesn’t sense anything wrong within the body, so there are mostly no symptoms at first. The infected person however can spread infected cells and viruses to other unsuspecting individuals through coughing and sneezing, helping the virus reach more victims in the process. The incubation period varies depending on the causative microbe, but for most respiratory infections it usually lasts between 1-2 days and a week.²

Prodromal Stage

Soon after enough viruses have been produced inside initially infected cells, some of them start to degrade and the resultant viral remnants leak out to the cell surface. Immune cells that are constantly scouring for infections sense these new patterns on the mucosa and begin to produce inflammatory molecules to control the infection. While the immune system does its best effort to stop the infection, this causes collateral damage to the surrounding tissues and leads to the manifestation of local symptoms such as sneezing, and excessive nasal discharge.

Acute Stage

As the virus is multiplying, it starts jumping to new healthy cells nearby, increasing the scale of the affected tissue. If the immune system is unable to put a stop to the infection on time, it can spread to the lower respiratory tract. While in the prodromal stage, a person can experience symptoms that can’t specifically be attributed to illness but also other conditions like allergies, symptoms in the acute stage, are a dead giveaway for illness. Such symptoms include a chesty cough, swollen lymph nodes, fever, and fatigue. A person is still contagious during the acute stage of disease. The duration of the acute stage is mostly dependent on a person’s immune system - in people with a strong immune system, the acute stage can practically be avoided and illness can pass with just a stuffy nose. In immunocompromised people, however, the acute stage can be excessively long, with the manifestation of severe symptoms and post-disease complications.

Convalescent Stage

In the following convalescent stage, the immune system finally starts overwhelming the infection, and by working its magic cocktail of antibodies, immune cells, cytokines, and the complement system, symptoms start to improve. The colour, thickness and volume of sputum decreases, fever starts to break, a person regains their appetite, and starts feeling better overall. But, the infection is still not gone and a person during the convalescent stage continues to be contagious. Moreover, a person’s immune system often is overworked and weakened, which can lead to the development of secondary bacterial infections, including severe pneumonia.

Recovery Stage

After the immune system has cleared the infection completely, symptoms go away and the affected person can no longer transmit the infection to others. Sometimes, people can develop complications after an infection has resolved.

What can you do to prevent complications?

Most viral respiratory infections go away on their own. Most healthy people can manage their illness by following a few simple rules:

• Rest well: rest and sleep help the body recover from infections more quickly
• Eat well: a diet rich in proteins, fats, vitamins and minerals, and low in simple sugars, can help boost immunity and shorten the duration of infections
• Stay hydrated: people should drink plenty of water or tea to help the body produce healthy mucus and the kidneys to filter out toxins more quickly
• Stay active: people with good fitness are less likely to have severe symptoms during the acute stage of infection and to develop complications after the infection has resolved

Symptoms of respiratory infections can be treated at home using over-the-counter (OTC) medications:

• Fever: fever is a physiologic reaction of the body to infections, and is important for the normal function of the immune system. For low to moderate fever under 38.5 C, many healthcare providers don’t recommend patients to take fever-reducing medications, unless a person is prone to fever-induced seizures or the fever causes unbearable discomfort. Fever is usually treated using OTC medications like acetaminophen (paracetamol) or ibuprofen. Acetylsalicylic acid should not be used in children with viral infections due to the risk of developing Reye’s syndrome which can severely damage the child’s liver and brain
• Cough and congestion: dry irritating cough can be treated with antihistamines or other cough suppressants, while chesty cough is treated with expectorants and mucoactive agents, as well as steam inhalation. A healthcare provider should assess the type of cough a person has - whether dry or chesty, as using the wrong type of cough medication can lead to a worsening of the infection. Nasal and sinus congestion can be relieved with the help of antihistamines and decongestants such as oral pseudoephedrine and nasal sprays or drops containing drugs like xylometazoline

Since most medications that are used for viral respiratory infections are dispensed over-the-counter (OTC), patients should actively seek advice from pharmacists on how to self-medicate most effectively and safely. Patients tend to take multiple medications containing the same active ingredients at once, which puts them at high risk of overdosing. For example, acetaminophen overdose is the most common cause of acute liver failure;³ chronic overuse of nasal decongestants can cause rebound congestion and permanent damage to the nose; some OTC antihistamines can induce sleepiness and should be avoided when driving.

When should you seek professional medical evaluation?

Most viral respiratory infections go away on their own, especially if a person has a strong immune system. In some cases, however, an infection can dramatically worsen with potentially life-threatening complications, and that’s when a person should seek medical help. Infections that initially affect the upper respiratory tract can progress to a lower respiratory tract infection with more severe symptoms. COVID-19 is well known for its pulmonary complications, including severe lung inflammation leading to pneumonia, and acute respiratory distress syndrome,⁴ which is considered the most severe COVID-19 complication and can lead to serious breathing problems needing respiratory support with mechanical ventilation and oxygen therapy.⁵ In case a person develops a secondary bacterial infection after the initial viral one, symptoms can start to worsen again after a period of improvement. Secondary bacterial pneumonia is a common complication of influenza and is a common reason why physicians can prescribe antibiotics during a viral infection.⁶ In people with a compromised immune system, the initial respiratory infection can also progress into a systemic infection, and may involve multiple organs outside the lungs:

• Infection of the brain (encephalitis) and surrounding membranes (meningitis) - symptoms include light sensitivity, neck stiffness, and severe headache
• Infection of the kidneys (nephritis) - symptoms include foamy urine, blood in the urine, and pelvic pain
• Infection of the heart muscle (myocarditis) - symptoms include chest pain, shortness of breath, and palpitations
• Infection of the bones (osteomyelitis) - symptoms include bone pain and swelling
• Generalised infection or sepsis - symptoms include low blood pressure, severe fever, lightheadedness, and changes in mental state

Many fatal cases of influenza complications have been documented, including rapidly progressive myocarditis,^7,8,9 encephalitis,^10,11 and sepsis.¹² If a person starts experiencing breathing problems or any extrapulmonary symptoms, they should immediately contact a physician, as these complications can quickly become fatal.

While the body usually recovers completely from respiratory infections, there are cases in which people have developed post-infection complications, some of which cause irreversible damage to the body. The most prominent example is the long-term complication of COVID-19 known as post-COVID syndrome or long COVID. Long COVID impacts multiple organs and systems throughout the body, being associated with autoimmune reactions, excessive blood clotting, cognitive decline, heart disease and many others. The good thing is that vaccines can prevent both acute disease and post-disease complications associated with viral infections.¹³

Conclusion

Viral respiratory infections are incredibly common, and it is practically impossible to avoid them completely. Younger people who stay fit and have a healthy immune system generally recover more quickly from infectious diseases. Vaccines can help prevent severe symptoms and complications, especially in high-risk populations. People should not over-self-medicate during viral respiratory infections, as they can cause more harm than good, and should seek medical help if any abnormal and severe symptoms occur. 

FAQ’s

Should I self-medicate with antibiotics?

No, this is a common mistake patients make for simple respiratory infections. Viruses cannot be killed by antibiotics, and apart from disrupting the microbiome and increasing the risk of antibiotic resistance, excessive use of antibiotics for viral infections does no good. Many physicians prescribe antibiotics in cases of severe viral respiratory infections when they suspect secondary bacterial infections. This practice should be guided by prior antibiotic resistance testing. Please, refrain from self-medicating with antibiotics. What you can do instead is to proactively ask your family doctor for the annual influenza and pneumococcal vaccines, to prevent respiratory diseases from happening in the first place.

If I have more severe symptoms, does that mean that my immune system is fighting the infection more effectively?

Yes and no. From what we have discussed above, you can conclude that symptoms of infection such as fever and mucus production, are physiologic reactions of the body that help fight infections. While this is correct, sometimes symptoms can be so severe that the immune system gets exhausted quickly and the body becomes an easy target for secondary infections. So you should seek medical attention should you develop severe symptoms at any stage of the disease.

Can boosting my immune system prevent severe complications of respiratory infections?

Contrary to mass commercial interests, in healthy individuals, there are not so many supplements that boost the immune system against infections. What has proven to work, are the good old vitamins and minerals. Vitamin C and vitamin D, as well as magnesium and zinc, are the most studied and amongst the most widely prescribed supplements for the immune system. They play important roles in the immune system as cofactors for immune cell maturation, production of antimicrobial substances, and protection of healthy tissues from collateral damage. It is important to adhere to the recommended dosage for each supplement, as it is fairly common for people to overdose on food supplements, especially fat-soluble vitamins (vitamins A, D, E, and K).

Are there any symptoms of worsening that are specific to young children or the elderly?

Yes, in infectious diseases children often are unable to express their exact condition, so parents and caregivers should be wary of unusual changes in behaviour which can be a sign of infection worsening. These include mood changes, irritability and unusual calmness. On the other hand, elderly people can experience worsening already pre-existing medical conditions like hypertension, heart failure, and cognitive decline. These two age groups are also at high risk of complications and healthcare providers should be vigilant of warning signs.