The vast majority of upper respiratory infections (URIs) are viral. In studies testing patients with flu-like symptoms and fever, about 82% of infections were caused by viruses alone, roughly 12% by bacteria alone, and the remaining cases involved both. So if you have a cold, sore throat, or sinus congestion, the odds strongly favor a virus as the culprit.
Which Viruses Cause Most URIs
Rhinovirus is the dominant player, responsible for 50% to 80% of common colds. It has a short incubation period of one to two days and typically causes nasal congestion, runny nose, sneezing, sore throat, and general fatigue. Fever is absent or low-grade. Symptoms usually last 7 to 10 days.
Other viruses round out the list: coronaviruses (incubation around 3 days), influenza (as short as 12 hours for type B), respiratory syncytial virus or RSV (about 4 days), adenovirus (5.5 days), parainfluenza, enterovirus, and human metapneumovirus. Influenza, RSV, and coronaviruses tend to peak in winter months, while rhinovirus circulates year-round with spikes in fall and spring.
When Bacteria Are the Cause
Bacterial URIs are less common but do happen. The bacteria most often involved include Streptococcus pyogenes (the group A strep behind strep throat), Streptococcus pneumoniae, Haemophilus influenzae, Moraxella catarrhalis, and Mycoplasma pneumoniae. These tend to produce more localized, intense symptoms compared to the diffuse achiness of a viral cold.
Strep throat is the bacterial URI people encounter most often. It typically comes on suddenly with a severe sore throat, fever, and swollen lymph nodes, but usually no cough, runny nose, or hoarseness. That pattern is the opposite of a viral cold, which tends to start gradually and includes congestion, coughing, and a scratchy (rather than severe) throat.
How a Viral URI Becomes a Bacterial One
One reason this question gets confusing is that viral infections can set the stage for a secondary bacterial infection. When a virus damages the lining of your airways, it kills cells that normally sweep bacteria out and exposes new attachment points where bacteria can latch on. The virus also dials down parts of your immune response, particularly the signaling pathways that recruit infection-fighting white blood cells to the area. The result: bacteria that normally live harmlessly in your nose or throat can multiply and cause a new infection on top of the original one.
This is the classic scenario behind a cold that “turns into” a sinus infection or bronchitis. The initial virus does the damage, and bacteria move in during the recovery window.
Symptom Timeline: Viral vs. Bacterial
The most reliable way to distinguish a viral URI from a bacterial one isn’t any single symptom. It’s the pattern over time.
A straightforward viral cold follows a predictable arc: symptoms build over two to three days, peak around day three or four, then gradually improve. By day 7 to 10, you’re mostly better. Viral pharyngitis (sore throat) tends to come on more gradually and often includes a runny nose, cough, and hoarseness.
Bacterial involvement is suspected when that arc breaks. Clinical guidelines point to three red flags in the timeline:
- No improvement after 10 days. If cold symptoms persist at the same level beyond 10 to 14 days, a bacterial sinus infection becomes more likely.
- Worsening after 5 to 7 days. You were getting better, then suddenly got worse. This “double sickening” pattern suggests bacteria have taken hold.
- Severe onset. A fever of 102°F (39°C) or higher, significant facial pain, and thick discolored nasal discharge appearing right away point more toward bacterial sinusitis than a typical cold.
Does Mucus Color Tell You Anything?
This is one of the most persistent beliefs in medicine, and it’s mostly wrong. Yellow or green mucus does have a statistical correlation with bacterial infection, but it’s so weak that it’s essentially useless for making decisions. In one study of patients with acute cough, only 12% of those tested had a confirmed bacterial infection. Green or yellow mucus caught about 79% of those bacterial cases (decent sensitivity), but it also flagged more than half of the non-bacterial cases as potentially bacterial (specificity of just 46%). The positive predictive value was only 16%, meaning that out of every 100 people with colored mucus, roughly 84 didn’t have a bacterial infection at all.
The color comes from enzymes released by your white blood cells as they fight any infection, viral or bacterial. So green mucus means your immune system is active. It does not mean you need antibiotics.
How Bacterial URIs Are Confirmed
For sore throats, the rapid strep test is the most common diagnostic tool. A large review of over 58,000 participants found these tests have a sensitivity of about 86% and a specificity of 95%. That means they catch most true strep cases and rarely give a false positive. If the rapid test is negative but strep is still strongly suspected, a throat culture (which takes one to two days) is more definitive.
For sinus infections and bronchitis, there’s no quick bedside test equivalent. Diagnosis relies heavily on that symptom timeline. This is part of why so many bacterial sinus infections are over-diagnosed, and why guidelines emphasize patience before prescribing antibiotics.
Why Antibiotics Usually Don’t Help
Because the overwhelming majority of URIs are viral, antibiotics won’t shorten the illness or reduce symptoms in most cases. Antibiotics kill bacteria; they do nothing against viruses. The CDC recommends against routine antibiotic use for the common cold, non-specific URIs, and acute bronchitis regardless of how long the cough lasts.
Even for confirmed bacterial sinus infections, current CDC guidance supports “watchful waiting” in uncomplicated cases where follow-up is available. Many bacterial sinus infections resolve on their own, and giving your body a few extra days can spare you the side effects of unnecessary medication, including disruption to your gut bacteria, allergic reactions, and contributing to antibiotic resistance at a population level. Most adults get two to four colds per year, so the cumulative impact of treating each one with antibiotics would be enormous.
Strep throat is the major exception. Antibiotics are recommended to prevent rare but serious complications affecting the heart and kidneys, and they also shorten how long you’re contagious.
Symptoms That Need Urgent Attention
Most URIs resolve without medical care, but certain symptoms suggest something more serious is happening. Seek emergency care for a fever over 103°F (40°C), difficulty breathing, chest pain, wheezing or stridor (a high-pitched sound when breathing in), dizziness, confusion, or visible skin pulling inward between your ribs with each breath. Contact your doctor if symptoms are severe, last longer than two weeks, or keep coming back frequently.