An infection can become an emergency with or without antibiotics

Seek urgent medical help for confusion, severe difficulty breathing, blue or grey colour, fainting, extreme pain, rapidly worsening illness, very low urine output, or fever or feeling very cold with clammy skin. Sepsis is the body’s life-threatening response to infection and requires fast assessment. Call emergency services for swelling of the tongue or throat, severe breathing difficulty, collapse, or a widespread blistering or peeling rash after a medicine.

Antibiotics treat selected infections caused by bacteria. They do not treat viruses, and even some bacterial infections improve without them. The right decision depends on the likely site and cause, severity, personal risk, examination, local resistance, and sometimes a test. Using an antibiotic only when it is likely to help protects the individual from avoidable harm and preserves treatment for the future.

Key points

Colds, influenza, and most acute coughs and sore throats are viral; antibiotics do not shorten them.
Green or yellow mucus does not by itself prove a bacterial infection.
A positive test must be interpreted with symptoms because some bacteria can be carried without causing disease.
If prescribed, take the antibiotic exactly as directed; never share it or save it for another illness.
Resistance develops in microbes, not because a person’s body becomes “used to” an antibiotic.

Bacteria and viruses are different

Bacteria are single-celled organisms. Many live harmlessly on the skin, in the gut, and elsewhere, and some support normal health. Certain bacteria can cause disease by invading tissue, producing toxins, or triggering inflammation. Antibiotics target structures or processes in susceptible bacteria.

Viruses are smaller infectious agents that reproduce inside host cells. Antibiotics do not block viral replication. Antiviral medicines exist for selected viruses, but they are a different group and are often useful only for particular people or at a particular stage.

Fungi and parasites are different again and need their own treatments. “Antimicrobial” is the broad term covering antibiotics, antivirals, antifungals, and antiparasitic medicines. The cause matters because one category cannot be substituted for another.

Symptoms do not always reveal the cause

Fever, cough, sore throat, swollen glands, pain, fatigue, and mucus can occur in both bacterial and viral infections. Thick, yellow, or green nasal discharge is common during viral colds because immune cells and proteins change its colour. Duration and pattern can shift probability, but no single symptom proves which microbe is responsible.

Location and context help. Strep throat is a bacterial condition for which testing and antibiotics may be appropriate. Most sore throats are not strep. A urinary infection can be bacterial, while burning can also come from an STI, vaginal condition, stone, or irritation. Pneumonia can be bacterial, viral, mixed, or non-infectious.

Some bacterial infections do not require antibiotics. CDC notes that many sinus infections and some ear infections improve without them. Conversely, a severe bacterial infection such as sepsis may require immediate antibiotics before every result is available. Stewardship means matching urgency and treatment to risk, not avoiding antibiotics at all costs.

How clinicians decide

The clinician considers age, pregnancy, immune status, symptoms, duration, severity, exposure, vaccination, vital signs, examination, and local disease patterns. A person who is immunocompromised, very young or old, recently hospitalized, or medically fragile may need a different threshold for testing or treatment.

Tests can include a rapid antigen or molecular test, urine test, culture, imaging, or blood work. A culture can identify a bacterium and which antibiotics are likely to work, but it takes time and may detect colonization rather than disease. A negative rapid test may not exclude every infection. Test quality, specimen collection, and pre-test probability all matter.

Possible decisions include no antibiotic with symptom care, a delayed prescription to use only if defined criteria develop, immediate treatment, or urgent hospital assessment. A useful plan states the expected course, what to do for symptoms, when to review, and which warning signs override watchful waiting.

What antibiotics can and cannot do

For a susceptible bacterial infection, an antibiotic may shorten illness, prevent complications, reduce transmission, or save life. The size of benefit differs. Treating confirmed strep throat is not the same decision as treating an uncomplicated cough.

An antibiotic will not make a viral cold resolve faster. Any improvement after starting one does not prove it worked; many infections naturally begin to improve at the same time. Unnecessary treatment still exposes the person to side effects and changes the microbiome.

“Strong” is not a useful way to choose an antibiotic. A broader-spectrum medicine affects more types of bacteria but may create more collateral damage and resistance pressure. The best option is the narrowest effective treatment for the likely or confirmed infection, using local guidance and patient factors.

How to use a prescribed antibiotic safely

Confirm what infection is being treated and whether a test result is pending.
Tell the prescriber about allergies, pregnancy, kidney or liver disease, and all medicines and supplements.
Follow the label for dose timing, food, storage, and course length.
Ask what to do if a dose is missed; do not automatically double the next one.
Do not share antibiotics or take medicine prescribed for another person.
Do not save leftovers for the next cough, sore throat, or urinary symptom.
Use a pharmacy take-back route when available rather than flushing or keeping unused medicine.

The slogan “always finish the course” can be misunderstood as ignoring new instructions or serious harm. The safe rule is to take the medicine exactly as currently prescribed. Do not stop early, extend the course, or reuse leftovers independently. If test results change the diagnosis, side effects occur, or the prescriber contacts you with a new plan, follow that updated clinical advice.

Side effect, allergy, and severe antibiotic-associated diarrhea

Nausea, mild diarrhea, abdominal discomfort, headache, or yeast overgrowth can occur with some antibiotics. These do not automatically mean allergy. Contact a pharmacist or prescriber for advice on severity, hydration, interactions, and whether treatment should change.

Hives, facial swelling, wheeze, throat tightness, or collapse may indicate immediate allergy. Severe delayed reactions can cause widespread rash, blisters, skin pain or peeling, mouth or eye sores, fever, facial swelling, or organ symptoms. These require urgent assessment; anaphylaxis is an emergency.

Clostridioides difficile, or C. diff, can cause diarrhea and inflammation of the colon during or after antibiotic use. CDC advises contacting a healthcare professional if diarrhea develops while taking or after taking an antibiotic. Frequent watery diarrhea, fever, abdominal pain, dehydration, blood, or severe weakness increases urgency. Do not self-treat significant diarrhea with an anti-diarrheal product before clinical advice.

What antibiotic resistance means

Resistance occurs when bacteria survive exposure to an antibiotic and pass on or acquire traits that reduce its effect. The person does not become resistant; the bacteria do. Resistant organisms can spread among people, animals, food, healthcare environments, water, and across borders.

Any antibiotic use creates selection pressure, even when treatment is appropriate. Misuse and overuse add avoidable pressure: using antibiotics for viruses, choosing an unnecessarily broad agent, wrong dosing, poor-quality medicines, and unregulated use in people or agriculture. Poor infection control, limited diagnostics, inadequate sanitation, and lack of access to the right treatment also drive the problem.

WHO reported in July 2026 that roughly one in six laboratory-confirmed bacterial infections worldwide in 2023 was resistant to antibiotics. Resistance can turn a routine infection or operation into a more complex, expensive, or dangerous event and threatens cancer care, transplantation, intensive care, and surgery.

Stewardship protects today’s patient too

Antimicrobial stewardship is a system for using these medicines judiciously so each patient gets effective treatment with the least avoidable harm. NICE includes guideline-based prescribing, diagnostic support, delayed prescriptions when appropriate, documentation, review, and monitoring resistance.

WHO groups antibiotics as Access, Watch, and Reserve. Access antibiotics are generally preferred first choices for many common infections when indicated; Watch antibiotics have higher resistance potential and need closer stewardship; Reserve medicines are protected for selected multidrug-resistant infections. The categories are not a shopping list and do not replace local clinical guidelines.

What individuals can do

Stay up to date with recommended vaccines, wash hands at appropriate times, prepare food safely, practise safer sex, and follow infection-control advice. These steps prevent infections and therefore reduce the need for antibiotics.

At an appointment, ask: “What is the most likely cause?”, “Would a test change treatment?”, “What can relieve symptoms?”, “If we wait, what should trigger review?”, and “What side effects require help?” It is reasonable to want a plan even when an antibiotic is not recommended.

Do not buy antibiotics from an unverified website or use products intended for animals. Counterfeit, degraded, contaminated, or incorrectly dosed medicine can delay diagnosis and cause harm.

When infection needs urgent assessment

Seek prompt care for worsening after initial improvement, persistent high fever, dehydration, severe localized pain, a spreading red or swollen area, pregnancy with infection symptoms, symptoms in a newborn, or illness in someone with major immune suppression.

Sepsis is a medical emergency. CDC lists confusion or disorientation, clammy or sweaty skin, extreme pain or discomfort, fever or shivering or feeling very cold, high heart rate or weak pulse, and shortness of breath among possible signs. A person does not need every sign. Say that you are concerned about sepsis and seek emergency help.

Questions people often ask

Does green mucus mean I need antibiotics?

No. Colour can change during viral illness. Duration, severity, examination, and sometimes testing determine whether a bacterial infection is likely.

Can a bacterial infection get better without antibiotics?

Yes, some do. Others require prompt treatment. The site, severity, complication risk, and patient factors determine the safest approach.

Can I use leftovers if the symptoms are identical?

No. Similar symptoms can have a different cause, the medicine or course may be wrong, and partial treatment can delay correct care.

Does taking antibiotics weaken my immune system?

Antibiotics do not generally “switch off” immunity, but they can disrupt helpful microbes and cause side effects. Unnecessary use offers risk without benefit.

Sources and evidence scope

This guide was researched using the WHO fact sheet on antimicrobial resistance (July 2026) and the AWaRe framework, current CDC guidance on antibiotic use, sepsis, and C. diff, and NICE guideline NG15 on antimicrobial stewardship. Sources were checked on August 21, 2026. Resistance patterns, prescribing rules, tests, and recommended treatments vary by country and locality.