Is my insect bite infected?
A normal bite reaction peaks within 48 hours and then improves. Infection does the opposite: it starts improving, then worsens after day two or three, with spreading warmth, increasing pain, pus or fever. That change of direction is the single most useful sign.
Normal reaction vs infection
| Normal | Infection |
|---|---|
| Worst at 24–48 hours, then better | Worsens after day 2–3 |
| Itchy | Increasingly painful |
| Redness stays roughly put | Redness spreads outward |
| Skin warm at most | Skin hot to touch |
| No fever | Fever, chills, feeling unwell |
The signs that mean be seen today
- Redness spreading beyond the original bite, especially if you can mark the edge with a pen and see it move within hours.
- A red streak running from the bite towards the body.
- Pus, or a wound that weeps and crusts repeatedly.
- Swollen, tender glands in the armpit or groin on the same side.
- Fever or chills.
- Increasing pain out of proportion to the appearance — this can indicate a deeper infection.
Why bites get infected
Almost always because they were scratched. Fingernails carry staph and strep, and a bite is an open door. Children are at higher risk simply because they scratch in their sleep.
Some bites are more prone than others: horsefly bites, because the mouthparts cut rather than pierce; and any bite that blisters and then bursts.
What helps
Wash with soap and water, keep nails short, and treat the itch early and properly — a cold pack for ten minutes, an oral antihistamine, and hydrocortisone cream if needed. Covering a scratched bite with a plaster overnight is a simple and effective intervention in children.
Antiseptic creams have a limited role. Once infection is established, oral antibiotics are usually needed, which means being seen.
What is not infection
A large, hot, swollen area around a wasp or bee sting appearing within hours is a large local reaction, not infection — it arrives too fast. Infection takes days. Similarly, the intense flare of a chigger or no-see-um cluster is inflammation, not bacteria.
Common questions
How fast does a bite infection develop?
Usually 48 hours to several days after the bite. Anything that appears within a few hours is a reaction, not an infection.
Should I put antibiotic ointment on every bite?
No. Routine use is unnecessary and can cause contact dermatitis. Wash, protect from scratching, and watch.
What if a red line is spreading from the bite?
That warrants same-day medical assessment.
Get a photo assessment in the app
Everything above is free to read. The app adds what a page cannot: point your camera at the bite and get an assessment of the most likely cause, a severity read, and a day-by-day care plan — including the specific signs that mean you should be seen.
iPhone and iPad. Free to download.