What Is Actually Happening in Your Body During a Cold

On the third day the mucus turns yellow, then green, and it looks like proof that something bacterial has moved in. It is not: that color is an ordinary phase of an ordinary cold. Most of what a cold feels like — the fever, the aching, the streaming nose — is your own immune response at work, which is why it follows a fairly predictable arc.

When to see a doctor

Right away, by emergency number — 911 in the US and Canada, 999 in the UK, 112 across the EU, 112 or 103 in Russia. In an adult: trouble breathing; persistent chest or abdominal pain or pressure; ongoing dizziness or confusion; being impossible to wake; seizures; no urine; severe muscle pain or weakness; a fever or cough that improved and then returned or worsened; a chronic condition flaring. In a child: fast or labored breathing with the ribs drawing in; blue lips or face; chest pain; refusing to walk because of muscle pain; dehydration — no urine for eight hours, dry mouth, no tears; not waking or reacting normally; seizures; a fever above 40 °C that will not come down; any fever in a baby under twelve weeks; getting worse after getting better. The NHS (UK) adds a stiff neck, a rash that does not fade under a pressed glass, pain in bright light, a first febrile seizure, unusually cold hands and feet, mottled or bluish skin, and a child hard to rouse or extremely agitated. Neither list is exhaustive.

The same day: fever longer than four days (CDC, US) or three (NHS, UK); symptoms past ten days without improvement; severe facial pain or headache; signs of dehydration. In a child also 38 °C under three months (NHS, UK), 39 °C from three to six months, five days or more of fever, a rash with a fever, refusing food and drink, dry diapers and sunken eyes.

At a routine appointment: a cough past three weeks; sinus infections that keep returning; a chronic illness or weakened immunity, worth planning for the season.

What is happening

The arc. CDC (US) describes a cold as usually lasting less than a week, the NHS (UK) expects improvement within one to two weeks, and in small children it runs longer. The cough is the straggler: it can last up to three weeks, and past three weeks it is worth checking.

The symptoms are the response, not the virus. Fever, aching, fatigue and a running nose come from the immune reaction — interferons, signaling molecules, immune cells arriving at the lining of the nose and throat — rather than from the virus dismantling tissue. That reframes what medicine is for: it changes how you feel, not who wins.

Color proves nothing. Yellow or green mucus around the second or third day is a normal phase of a cold. It is not a sign of bacteria and not a reason for an antibiotic.

The sinuses. Sinusitis after a cold is more often viral than bacterial, and most cases clear without antibiotics — the NHS (UK) notes it can take up to four weeks. What does deserve a call: severe facial pain or headache, getting worse after getting better, symptoms past ten days, or a fever lasting more than three to four days.

What to do

Treat a fever for comfort, not for the number. In an adult a fever means 38 °C or above; paracetamol or ibuprofen by the label help if you feel unwell, along with rest and enough fluid not to get dehydrated. In pregnancy, from about week 20 the FDA (US) advises against ibuprofen and other NSAIDs unless a doctor says otherwise; paracetamol is the usual choice.
For a child, paracetamol or ibuprofen if they are miserable — not alternated or combined without a doctor's word. No paracetamol under two months, no ibuprofen under three months or under 5 kg, none with chickenpox or with dehydration, and with asthma only on medical advice. Aspirin is off limits under 16 (NHS, UK; under 18 in the US). Do not undress, sponge or wrap up a feverish child; fever medicine does not prevent febrile seizures.
Give the cough time and do not read its length as a failure of treatment. What actually eases symptoms: what helps with a cold.
Do not ask for an antibiotic for a cold, whatever color the tissues are: antibiotics and viruses.

When this is not the case

Some people should not wait the arc out: adults 65 and older, children under five and especially under two, pregnant women, and anyone with chronic lung disease including asthma, heart disease, diabetes, chronic kidney disease, weakened immunity, a disability with other conditions, or a BMI of 40 or more. For them an early call is the right call, and what to do at the first signs of a cold sets out the rest. Rarely a bacterial infection such as pneumonia does follow the viral one — that is exactly where antibiotics belong.

Sources

Checked: September 6, 2026