Colds, Flu, and COVID-19

TL;DR. Three illnesses that share a route (the air you breathe) and are otherwise quite different. A cold is a mild infection of the nose and throat by one of hundreds of viruses, and you will catch new ones forever because there are too many to become immune to all of them. Influenza is a systemic illness that puts you in bed, kills hundreds of thousands of people a year, and mutates fast enough to require a new vaccine annually. COVID-19 is a coronavirus that entered humans in 2019, spread globally because people were infectious before they felt ill, and has settled into circulation as a recurring respiratory disease. Nearly everything you feel during all three is your immune response, not the virus.

Key takeaways

• Antibiotics do nothing for any of these. They are viral. Taking antibiotics provides only side effects and resistance.
• Influenza kills an estimated 290,000 to 650,000 people a year in ordinary seasons, concentrated in the very old, the very young, and people with chronic disease.
• The 1918 influenza pandemic killed on the order of 50 million people, more than the First World War, and it killed young healthy adults disproportionately.
• COVID-19 spread globally because of presymptomatic transmission. SARS in 2003 was contained because patients were most infectious after they were obviously sick. SARS-CoV-2 was not.
• Flu vaccine effectiveness varies year to year (roughly 20 to 60 percent against illness) because the strains are selected months in advance. Partial protection still prevents a great deal of severe disease.
• Long COVID is real and measurable, affecting a minority of infections, with fatigue, breathlessness, and cognitive symptoms lasting months.

What they are

In short: Three illnesses sharing a route and little else, and the table of differences is genuinely useful when you are trying to work out which you have.

RSV (respiratory syncytial virus) deserves separate mention: it is the leading cause of hospitalisation in infants worldwide and a significant killer of older adults, and it has only recently become preventable through maternal vaccination, infant monoclonal antibodies, and adult vaccines.

Don't be confused: "stomach flu" is not influenza. Vomiting and diarrhoea are usually norovirus or rotavirus, unrelated viruses that infect the gut. Influenza is a respiratory illness, though it can cause vomiting in children. The naming confusion leads people to dismiss real influenza as a stomach bug.

The history

In short: The 1918 pandemic killed on the order of 50 million people, and COVID-19 produced the fastest vaccine development in history.

Respiratory epidemics are ancient, and influenza's pattern of sudden citywide illness was described repeatedly from the 1500s onward; the name comes from Italian, from the belief that the illness was under the influence of the stars.

1918. An influenza A(H1N1) pandemic spread worldwide in three waves, killing an estimated 50 million people, possibly more, out of a world population of under 2 billion. It was unusual in killing healthy adults aged 20 to 40 at high rates, probably through an overwhelming immune response and secondary bacterial pneumonia in an era before antibiotics. It was called "Spanish flu" only because wartime censorship suppressed reporting elsewhere while neutral Spain reported freely.

Subsequent pandemics: 1957 (H2N2), 1968 (H3N2), and 2009 (H1N1pdm09), each milder than 1918 but each demonstrating the same mechanism, a virus with a surface protein combination the population had no immunity to.

The science: influenza virus was isolated in 1933; the first vaccines were developed in the 1940s; the global surveillance network that selects each year's strains was established in 1952 and now spans over a hundred countries.

Coronaviruses were considered minor cold viruses until 2002, when SARS emerged in southern China, infected about 8,000 people, killed roughly 10 percent of them, and was contained by isolation and contact tracing within months. MERS followed in 2012 from camels, with high lethality and poor human-to-human transmission.

COVID-19. SARS-CoV-2 was identified in Wuhan, China, in December 2019 and declared a pandemic in March 2020. It caused approximately 7 million confirmed deaths, with excess mortality analyses suggesting a true toll in the range of 15 to 20 million. It also produced the fastest vaccine development in history: from published viral sequence in January 2020 to authorised mRNA vaccines in December 2020, a compression of a process that normally takes a decade.

What actually goes wrong

In short: Each virus has a key that fits a specific lock on your cells, and nearly everything you feel is your immune response rather than the virus.

Entry. Every respiratory virus has a key that fits a lock on your cells. Influenza binds sialic acid residues on airway cells. SARS-CoV-2 binds ACE2, a receptor present in the nose, lungs, gut, and blood vessel lining, which explains why COVID-19 is more than a lung disease. Rhinovirus mostly binds ICAM-1 and replicates best at the slightly cooler temperature of the nose, which is why colds stay in the upper airway.

Replication and damage. The virus hijacks the cell's machinery to make copies, and the cell dies or is killed by immune cells. Loss of the ciliated cells that sweep mucus upward impairs clearance for days to weeks, which is part of why a cough lingers and why secondary bacterial infection can follow.

Symptoms are the response. Interferons and other cytokines produce fever, aches, and fatigue. Increased mucus and vascular leak produce a runny nose. Inflammation of airway nerves produces cough. This is why symptoms track the immune response rather than viral load, and why two people with the same virus can have very different illnesses.

Why influenza recurs. Antigenic drift is the steady accumulation of mutations in surface proteins, requiring annual vaccine updates. Antigenic shift is the sudden swap of whole gene segments between human and animal influenza viruses co-infecting one host, producing a virus the population has no immunity to. Shift causes pandemics; drift causes seasons.

Why COVID-19 succeeded where SARS failed. SARS-CoV-1 patients were most infectious several days into symptomatic illness, so isolating sick people worked. SARS-CoV-2 transmits substantially in the day or two before symptoms and from people who never develop them, which defeats symptom-based control. That single difference in timing explains two very different global outcomes.

Severe disease. In severe influenza and COVID-19, the alveoli fill with fluid and inflammatory cells, oxygen transfer fails, and the illness becomes acute respiratory distress syndrome. COVID-19 additionally damages the blood vessel lining and provokes clotting, producing pulmonary emboli, strokes, and heart attacks at rates well above other respiratory infections.

What it does to the body

In short: The complications matter more than the illness: secondary bacterial pneumonia, asthma and COPD attacks, heart attacks in the following weeks, and long COVID.

Beyond the illness itself:

• Secondary bacterial pneumonia, historically the main killer in influenza pandemics.
• Exacerbations of asthma and COPD, which is how these viruses kill many people with chronic lung disease.
• Myocarditis and increased heart attack and stroke risk in the weeks after infection, documented for both influenza and COVID-19.
• Croup in young children (parainfluenza), bronchiolitis in infants (RSV).
• Otitis media and sinusitis as complications of colds, particularly in children.
• Long COVID (post-COVID condition): fatigue, breathlessness, cognitive difficulty ("brain fog"), palpitations, and post-exertional worsening persisting months after infection. Estimates of frequency vary widely with definition, from a few percent to over 10 percent of infections, and it is more common after severe illness though it occurs after mild ones. Vaccination reduces the risk. Mechanisms under investigation include viral persistence, autoimmunity, microclotting, and nervous system dysregulation. Nothing is yet proven, and no established treatment exists.

Is it deadly?

• Colds essentially never kill healthy people, though RSV and other "cold" viruses do kill infants and frail older adults.
• Seasonal influenza: approximately 290,000 to 650,000 respiratory deaths a year globally, with the burden concentrated at the extremes of age. Pandemic influenza is a different order of magnitude and is regarded by public health agencies as one of the highest-probability catastrophic threats.
• COVID-19: about 7 million confirmed deaths and an estimated 15 to 20 million excess deaths through the acute pandemic period. Infection fatality risk falls steeply with age and rises steeply with immunosuppression and chronic disease, and it dropped substantially after widespread vaccination and infection-acquired immunity.

Is it contagious?

Yes, all of them, by the respiratory route: droplets and aerosols exhaled while breathing, talking, coughing, and singing, with fine particles accumulating in poorly ventilated indoor spaces. Contaminated hands and surfaces contribute, more for rhinovirus and RSV than for influenza or SARS-CoV-2.

Practical consequences of the physics: ventilation and air filtration are effective, crowding and time indoors increase risk, and outdoors is dramatically safer than indoors. Masks reduce both emission and inhalation, with effectiveness depending heavily on fit and type.

Who gets it

Everyone. Adults average two to four colds a year and young children considerably more, which is normal immune education rather than a sign of weakness.

Higher risk of severe disease: age over 65 (and, for RSV and influenza, under 2), pregnancy, chronic lung disease, heart disease, diabetes, obesity, kidney and liver disease, immunosuppression, and Down syndrome. In many countries, severe outcomes have also tracked occupation, housing density, and access to care.

Geographically, influenza is winter-seasonal in temperate zones and year-round or rain-linked in the tropics, which complicates vaccine timing.

Treatment, and how it works

In short: No antiviral for colds, modest ones for flu and COVID-19, and one cheap steroid that cut deaths in severe COVID-19 by up to a third.

Colds

No antiviral treatment exists or is needed. Rest, fluids, and symptom relief: paracetamol/acetaminophen or ibuprofen for aches and fever, saline nasal rinses, decongestants for a few days at most (longer causes rebound congestion), honey for cough in adults and children over one year (with reasonable trial evidence, and never for infants because of botulism risk). Zinc lozenges may shorten duration modestly. Vitamin C does not prevent colds in the general population and shortens them marginally at best.

Influenza

Neuraminidase inhibitors (oseltamivir, zanamivir) block the enzyme influenza uses to release new virus particles from infected cells. Baloxavir blocks a viral polymerase step. Both work best started within 48 hours, and their benefit in otherwise healthy adults is modest (roughly a day less illness). Their value is greater in people at high risk of complications and in hospitalised patients, where the evidence base is observational but consistent.

COVID-19

Nirmatrelvir/ritonavir blocks the viral protease required to cut viral proteins into working pieces. Started within five days of symptoms in high-risk patients, it substantially reduces hospitalisation. It interacts with many common medicines, which limits its use. Remdesivir is an alternative in some settings.

For severe disease, the most important discovery came from repurposing a cheap old drug. The RECOVERY trial, a pragmatic randomised trial run across UK hospitals during the pandemic, showed that dexamethasone, a corticosteroid costing a few dollars, reduced deaths in patients requiring oxygen by about a fifth and in those on ventilation by about a third. The same trial showed that hydroxychloroquine and lopinavir/ritonavir did not work, which stopped their use. It is the best modern demonstration that a well-run trial during a crisis is faster and more useful than confident improvisation.

Vaccines

• Influenza: reformulated annually from global surveillance. Effectiveness against illness ranges roughly 20 to 60 percent depending on the match, and protection against hospitalisation and death is better than against infection. Annual vaccination is recommended for older adults, pregnant women, children, and people with chronic conditions in most countries.
• COVID-19: mRNA vaccines deliver instructions for the spike protein in a lipid nanoparticle; the cell produces the protein and the immune system learns it. Protection against infection wanes within months; protection against severe disease is more durable. Boosters are targeted at those at highest risk.
• RSV: maternal vaccination in pregnancy protects newborns through transferred antibodies, a long-acting monoclonal antibody (nirsevimab) protects infants directly, and vaccines are approved for older adults.

What treatment costs

• Oseltamivir: nausea and vomiting are common. Its overall value in healthy adults has been debated since the full trial data were released after a long fight over access to them, which itself changed the rules on clinical trial transparency.
• Nirmatrelvir/ritonavir: metallic taste, and significant drug interactions through the liver enzyme pathways described in Chapter 16.
• Corticosteroids: raise blood sugar, increase infection risk, and cause neuropsychiatric effects. They help in severe COVID-19 and harm in early mild disease, where suppressing the immune response is exactly wrong.
• Vaccines: sore arm, fever, and fatigue for a day or two are the common effects. Myocarditis after mRNA vaccination occurs mainly in young males at a rate of roughly 1 to 10 per 100,000 doses, is usually mild and self-limiting, and is substantially less common than myocarditis caused by COVID-19 infection itself.

What the person can do

• Get vaccinated if you are in a risk group or live with someone who is. The strongest argument for influenza vaccination in a healthy 30-year-old is often the 80-year-old they visit.
• Ventilate. Open a window, use an air filter, meet outdoors when possible. This is the most underused intervention and it works against all of them.
• Stay home when ill, which is a policy question as much as a personal one: paid sick leave measurably reduces workplace transmission.
• Wash hands, especially for rhinovirus and RSV.
• Do not ask for antibiotics for a viral illness, and be sceptical when they are offered without a clear bacterial reason.
• Seek care for warning signs: difficulty breathing, chest pain, confusion, blue lips, inability to keep fluids down, or symptoms that improve and then sharply worsen, which often signals secondary bacterial infection.

Living with it

For most people these are episodic nuisances. Two groups experience something different. People with long COVID frequently report their symptoms dismissed, a pattern familiar from other post-infectious syndromes including myalgic encephalomyelitis/chronic fatigue syndrome, which follows various infections and has been under-researched for decades. The practical management that has evidence is pacing (staying within an energy envelope to avoid post-exertional crashes) rather than graded exercise escalation, plus treating specific components such as orthostatic intolerance.

And people who are immunocompromised live with a permanently different risk calculation, since vaccines work less well for them and community transmission that is a minor matter for others is not for them.

What's next

• Universal influenza vaccines targeting the conserved stem of the surface protein rather than its variable head, which would end annual reformulation.
• Pan-coronavirus vaccines, aiming at protection against future spillovers rather than current variants.
• Mucosal (nasal) vaccines, which could block infection and transmission rather than just severe disease, the main gap in current injected vaccines.
• Long COVID trials of antivirals, immune modulators, and anticoagulants, currently the field's largest open problem.
• Better indoor air standards, treating clean air as a building requirement in the way clean water became one in the nineteenth century.

Sources and notes

Influenza mortality range (290,000 to 650,000 respiratory deaths annually) is WHO's estimate. COVID-19 confirmed deaths are WHO surveillance data; excess mortality estimates of roughly 15 to 20 million are from WHO and The Economist modelling and remain uncertain. 1918 pandemic death estimates range from 20 to 100 million; 50 million is the most commonly cited central figure. RECOVERY dexamethasone results: NEJM, 2021. Vaccine effectiveness ranges are from US CDC and European network annual estimates. Myocarditis rates after mRNA vaccination: multiple national surveillance analyses, 2021 to 2023. Long COVID prevalence estimates vary by definition and study design and are given as a range for that reason. Rhinovirus type counts are from genotyping surveys.

Open questions. The biological basis of long COVID is unresolved and no treatment is established. Whether SARS-CoV-2 will settle into a stable seasonal pattern is not yet clear. The origin of SARS-CoV-2 remains under investigation.

Next: what happens when a respiratory infection reaches the alveoli, and what happens when the body's response to infection becomes the thing that kills you. 👉