✓ Medically reviewed by · Last reviewed: May 2026
Pharmacy Researcher · 8 years experience
Pharmacy researcher with 8 years reviewing clinical drug information, generic formulation equivalence, and international pharmaceutical standards. Focuses on patient-facing accuracy in medication education.
Key Takeaways
- Bacterial infections are caused by pathogenic bacteria — distinct from viral infections, which antibiotics cannot treat.
- The most common types are UTIs, respiratory tract infections, skin infections, STIs, and GI infections.
- Treatment always depends on the specific bacteria and infection site — different antibiotics cover different bacterial spectra.
- Antibiotic resistance is a growing problem; taking the wrong antibiotic, or stopping a course early, accelerates resistance.
- Many bacterial infections can be treated with oral antibiotics ordered online — see our complete guide to ordering antibiotics online.
Bacterial infections are one of the most common reasons people seek medical treatment — from urinary tract infections and chest infections to skin conditions and sexually transmitted infections. Yet bacterial and viral infections are often confused, and that confusion leads to misuse of antibiotics (overuse in viral illness, underuse or wrong antibiotic in bacterial illness).
This guide covers what bacterial infections are, how to tell them from viral infections, the most common bacterial infection types, how they are treated, and when it is appropriate to treat with antibiotics ordered online.
What is a bacterial infection?
A bacterial infection occurs when pathogenic (disease-causing) bacteria enter the body, multiply, and trigger an immune response. Bacteria are single-celled microorganisms. Most bacteria are harmless or beneficial (the gut microbiome, for example, contains trillions of bacteria essential to health). Pathogenic species cause harm either by directly damaging host tissue or by releasing toxins.
Unlike viruses, bacteria are living cells that can be targeted by antibiotics — compounds that either kill bacteria directly (bactericidal) or prevent their reproduction (bacteriostatic).
Bacterial infection vs viral infection — key differences
| Feature | Bacterial infection | Viral infection |
|---|---|---|
| Causative agent | Living bacteria cells | Viral particles (not living) |
| Respond to antibiotics? | Yes | No |
| Typical onset | Often localised + gradual | Often systemic + abrupt |
| Fever pattern | Persistent; may worsen | Often peaks then breaks |
| Discharge/secretions | Often thick, coloured, purulent | Often clear or watery |
| Duration without treatment | May worsen; rarely self-resolves | Often self-limiting 7–14 days |
The key clinical rule: antibiotics treat bacterial infections, not viral ones. Taking amoxicillin for a viral cold does not shorten the illness; it exposes you to side effects and contributes to antibiotic resistance.
Most common bacterial infections
1. Urinary tract infections (UTIs)
Escherichia coli causes approximately 80–85% of uncomplicated UTIs. Women are disproportionately affected. Symptoms include burning on urination, urgency, frequency, and sometimes cloudy or foul-smelling urine.
First-line antibiotics: nitrofurantoin (3–7 days), trimethoprim (7 days), fosfomycin (single dose). For complicated UTIs or those caused by resistant organisms: ciprofloxacin or co-amoxiclav.
2. Respiratory tract infections (RTIs)
Most upper respiratory infections (colds, most sore throats) are viral. However, bacterial pneumonia, bacterial sinusitis, whooping cough (Bordetella pertussis), and bacterial tonsillitis (Streptococcus pyogenes) are legitimately bacterial and antibiotic-responsive.
First-line antibiotics: amoxicillin or amoxicillin-clavulanate for most community-acquired RTIs; azithromycin or doxycycline for atypical pathogens (Mycoplasma, Chlamydophila).
3. Skin and soft tissue infections (SSTIs)
Cellulitis, impetigo, folliculitis, and infected wounds are commonly caused by Staphylococcus aureus or Streptococcus pyogenes. Acne vulgaris involves Cutibacterium acnes and is treated with topical or oral antibiotics in moderate-to-severe cases.
First-line antibiotics: flucloxacillin or cefalexin for skin/soft tissue; doxycycline or minocycline for acne.
4. Sexually transmitted bacterial infections (STIs)
Chlamydia (Chlamydia trachomatis), gonorrhoea (Neisseria gonorrhoeae), and syphilis (Treponema pallidum) are among the most prevalent bacterial STIs globally. All are antibiotic-treatable when caught early.
First-line antibiotics: doxycycline or azithromycin for chlamydia; ceftriaxone + azithromycin for gonorrhoea (due to widespread resistance); benzathine penicillin or doxycycline for syphilis.
5. Gastrointestinal infections
Common bacterial GI pathogens include Salmonella, Campylobacter, Shigella, and Clostridioides difficile. Most self-resolve in healthy adults; antibiotic treatment is reserved for severe cases, immunocompromised patients, or high-risk pathogens.
Note: H. pylori — the bacteria causing most stomach ulcers — is treated with a “triple therapy” regimen: a proton pump inhibitor + two antibiotics (usually amoxicillin + clarithromycin, or metronidazole + clarithromycin).
6. Ear infections (otitis media)
Middle ear infections in children are commonly bacterial (Streptococcus pneumoniae, Haemophilus influenzae). Most resolve spontaneously in 48–72 hours; antibiotics (amoxicillin) are recommended for young children, bilateral infections, or those not improving.
How are bacterial infections diagnosed?
Definitive diagnosis requires a bacterial culture — a lab grows the organism from a sample (urine, blood, sputum, swab) and tests antibiotic sensitivity. However, most community bacterial infections are treated empirically (based on the most likely pathogens for that infection site and population) before culture results return. Empirical treatment follows local resistance patterns and clinical guidelines.
Diagnosis relies on:
- Clinical history and examination — symptoms, location, duration, risk factors.
- Dipstick urinalysis — nitrites and leucocyte esterase suggest UTI.
- Throat swab — rapid strep test or culture for bacterial tonsillitis.
- Chest X-ray — consolidation suggests bacterial pneumonia.
- Culture and sensitivity — gold standard, but takes 48–72 hours; used to guide treatment adjustment.
- Blood tests — elevated CRP, elevated WBC with neutrophilia suggest bacterial rather than viral cause.
Antibiotics by bacterial infection type
| Infection type | Common bacteria | First-line antibiotic | Duration |
|---|---|---|---|
| UTI (uncomplicated) | E. coli | Nitrofurantoin, trimethoprim | 3–7 days |
| Community-acquired pneumonia | S. pneumoniae, Mycoplasma | Amoxicillin or doxycycline | 5–7 days |
| Bacterial tonsillitis | Group A Strep | Amoxicillin, phenoxymethylpenicillin | 10 days |
| Chlamydia | C. trachomatis | Doxycycline 100mg twice daily | 7 days |
| Skin cellulitis | Staph aureus, Strep | Flucloxacillin or cefalexin | 5–7 days |
| H. pylori | H. pylori | PPI + amoxicillin + clarithromycin | 7–14 days |
| Acne (moderate–severe) | C. acnes | Doxycycline or minocycline | 3–6 months |
When can I order antibiotics online?
For well-characterised, self-limiting bacterial infections — uncomplicated UTIs, mild-to-moderate RTIs, travellers’ diarrhoea prevention, acne, or STI treatment with confirmed diagnosis — online antibiotic access is practical and widely used.
MedsBase stocks the most commonly prescribed antibiotics including amoxicillin, doxycycline, azithromycin, metronidazole, and ciprofloxacin. Full dosing guidance, indication breakdown, and ordering information is in our complete guide to ordering antibiotics online in 2026. For a comparison of the 10 most evidence-backed antibiotic choices by indication, see Best Antibiotics 2026.
- People who have had the same infection before and recognise the pattern (e.g., recurrent UTI)
- Travellers needing standby antibiotics for travellers’ diarrhoea or chest infections
- People managing acne long-term with a stable antibiotic regimen
- People in countries or situations where a GP appointment is not easily accessible
Medical disclaimer: This guide is for informational purposes. Seek emergency medical care for symptoms of severe infection: high fever above 39°C, severe chest pain, difficulty breathing, altered consciousness, or rapidly spreading redness/swelling. Antibiotics should be used only for confirmed or strongly suspected bacterial infections.
Frequently Asked Questions
What is a bacterial infection?
A bacterial infection occurs when pathogenic bacteria enter the body, multiply, and cause illness. Unlike viral infections, bacterial infections respond to antibiotic treatment.
How do I know if my infection is bacterial or viral?
Bacterial infections often cause localised symptoms, persistent or worsening fever, and purulent (thick, coloured) discharge. Viral infections often present with systemic symptoms, a fever that peaks and breaks, and clear secretions. Blood markers (elevated CRP, neutrophilia) support bacterial diagnosis. However, clinical assessment by a clinician is the most reliable method for ambiguous cases.
Can bacterial infections go away without antibiotics?
Some mild bacterial infections (mild ear infections, mild bacterial sore throat) may self-resolve. Most moderate-to-severe bacterial infections require antibiotics to clear fully and to prevent complications like abscess formation or systemic spread. Untreated UTIs, for example, can progress to kidney infection (pyelonephritis).
What is the strongest antibiotic for bacterial infection?
There is no single “strongest” antibiotic — the right choice depends on the infecting organism and the infection site. For serious gram-negative infections, carbapenems (meropenem, imipenem) are often considered the broadest-spectrum option. For community infections, amoxicillin-clavulanate, ciprofloxacin, or doxycycline cover most common pathogens.
Are bacterial infections contagious?
Many are. Respiratory bacterial infections (Group A Strep, Bordetella pertussis) spread via droplets. Bacterial STIs spread via sexual contact. Skin infections can spread by contact. Foodborne bacterial infections spread via contaminated food or water. Enteric infections spread via the faecal-oral route.
How long do bacterial infections last with antibiotics?
Most uncomplicated bacterial infections improve within 24–72 hours of starting the correct antibiotic. Full courses should always be completed (typically 5–14 days depending on the type) even if symptoms resolve — stopping early can allow resistant bacteria to survive and repopulate.
What antibiotic treats bacterial infections?
It depends on the infection type. Amoxicillin and amoxicillin-clavulanate are first-line for many RTIs and UTIs. Doxycycline covers a broad range of respiratory, skin, and STI bacteria. Azithromycin is used for atypical respiratory pathogens and chlamydia. Metronidazole covers anaerobic bacteria and H. pylori.
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Written by
Sophie ChenPharmaceutical Content Researcher · 8 years experience
Sophie Chen is a pharmaceutical content researcher with 8 years covering generic medication access and clinical pharmacology. She specialises in international regulatory frameworks, bioequivalence standards, and patient-facing education on therapeutic drug classes. She is not a clinician.