Is it a normal bite reaction or an infected bug bite?

Most bug bites itch, swell, and look red or pink for a day or two, then ease (Mayo Clinic). Redness on its own doesn't mean a bite is infected, and a big hive-like bump doesn't mean you're allergic. For the everyday insect bites in a children's hospital guide, the itch lasts several days, the redness usually fades by about day three, and the swelling can last up to a week (Children's Hospital Colorado).

Normal also covers a wide range of looks. A mosquito bite can be a puffy bump within minutes, a hard itchy reddish-brown bump the next day, a patch of small blisters, or a dark spot like a bruise (CDC). So how dramatic a bite looks tells you little. Which way it's heading tells you more: a bite that hurts more tonight than this morning, or a red patch that's wider on day three than on day two, is going the wrong way.

On brown or black skin, redness is harder to see. Run a fingertip over the area instead, because a bite still feels raised and swollen, and skin that feels hot is one of the signs the NHS lists for asking a pharmacist (NHS).

Yes. Bacteria need a way in, and a bite is a break in the skin, sometimes too small to see (NHS). Dry, flaky, or swollen skin lets them in most easily, and the usual culprits are strep and staph, with a tougher staph called MRSA becoming more common (Mayo Clinic). Scratching hard, or with something that breaks the skin, is one way a bite turns into an infection (Mayo Clinic). Some bites start out likelier to go wrong: horsefly bites are large, painful, and often become infected (NHS).

A children's hospital sorts bite infections into three kinds, listed below, and which one you have is a clinician's call (Children's Hospital Colorado).

• Impetigo: sores, soft scabs, and pus after scratching or picking, more common on itchy bites.
• Cellulitis: redness spreading out from the bite, painful to touch.
• Lymphangitis: a red line running up the arm or leg, taken more seriously because the infection can reach the bloodstream.

Pus, or skin around the bite that is hot, red, painful, and swollen, is the point where the NHS says to get a bite looked at, starting with a pharmacist for anyone age one or older (NHS).

One common version is impetigo. It starts as red sores or small blisters that burst and leave golden-brown crusty patches, sometimes after yellow liquid weeps out and hardens into a scaly edge around a scab. It itches, sometimes hurts, and often shows up on the face and hands (NHS). A bite you've scratched open is an easy place for it to start.

If a scab drains pus or keeps getting bigger but you feel fine, Children's Hospital Colorado treats that as a call during office hours, not an emergency (Children's Hospital Colorado). On any wound, pus, pain, and irritation all signal possible infection and the need for medical care, and cellulitis itself brings swelling, warmth, pain, fever, chills, blisters, or dimpled skin (Mayo Clinic).

Clear fluid is a different story from pus. A small water blister in the middle of a bite is a normal reaction, and it's common in younger children (Children's Hospital Colorado). A closed clear blister with no widening redness and no fever fits that. The blister or fluid-filled bump page shows what these look like.

Fire ant stings are the exception that trips people up. Their venom kills bacteria and some skin cells, so the blister fills with cloudy white material in about 24 hours. It looks like pus that should be drained, yet it's sterile and heals quickest if you leave it alone (American Academy of Allergy, Asthma & Immunology).

A big swollen mosquito reaction can also break open into an ulcer, especially if it's scratched, and then it can get infected or leave a scar. Cleveland Clinic puts broken skin that oozes pus, looks discolored, or smells bad on its call-911-or-go-to-the-ER list for that reaction (Cleveland Clinic). Whether yours has reached that point is for a clinician to judge, but a bad smell from an open bite is a good reason not to wait.

If the bite is already scratched open or scabbed, treat it like any small wound: wash it gently with soap and water, cover it with a bandage, and change the bandage at least once a day (Mayo Clinic). Keep fingers off any crust. Touching or scratching impetigo sores helps them spread and scar (NHS).

Children's Hospital Colorado suggests an over-the-counter antibiotic ointment on a scab that looks infected, and says redness or a red streak spreading from it means the child needs to be seen (Children's Hospital Colorado). Whether an ointment fits your bite is a question for a pharmacist or clinician, and a streak or spreading redness means being seen first.

Some bites swell widely, turn red and warm, and hurt, and still aren't infected. A large local reaction to mosquito saliva, called skeeter syndrome, causes redness, warmth, swelling, and itching, and the area can feel hard and painful, enough to make you wonder if you have a skin infection. It's most common in young children (Cleveland Clinic).

Timing is a clue. That reaction starts 8 to 10 hours after the bite and eases within 3 to 10 days (Cleveland Clinic). A strong reaction to a bee or other sting follows a similar pattern, with swelling that gets worse over a day or two and can last up to seven days (Mayo Clinic). Neither is a bacterial infection, but both can look like one.

Nothing I read gives a way to tell a big allergic reaction from early cellulitis by eye. Mayo Clinic says skeeter syndrome is sometimes mistaken for a bacterial infection, and there's no simple blood test to settle it (Mayo Clinic). A clinician looks at the timing and the skin. Large reactions are likelier in children, in adults bitten by a mosquito species new to them, and in people with immune system disorders (CDC). The swelling or large welt page shows what a big reaction can look like.

You can relax a little if the swelling has peaked and is shrinking, the pain is easing, the red patch isn't wider than it was yesterday, and the main complaint is itch.

These point the other way: new redness that starts more than 48 hours after the bite, redness still getting larger after 72 hours, or redness or a red streak bigger than 1 inch (2.5 cm). Those are the points where Children's Hospital Colorado says to contact a doctor within 24 hours (Children's Hospital Colorado). That guide is written for ordinary insect bites in children, so treat the numbers as a prompt to call, not a verdict. If the red area is widening, the guide to a bug bite that keeps spreading goes through it in detail.

To judge growth, take photos each morning and evening in the same light and note the date. They're easier to trust than memory, and a clinician can see the same series.

Cellulitis most often turns up on the lower leg, usually one leg only (Mayo Clinic). Red, swollen skin on both legs usually points to a different condition, so two matching red shins point to something other than a single bite. In children it tends to appear on the face or neck (American Academy of Dermatology).

On a hand, cellulitis shows up as swollen fingers or a swollen back of the hand (NHS), which is worth remembering if a bite near a knuckle is making the whole finger puffy. A baby under one year old with any bite or sting should be seen by a doctor (NHS).

A swollen rash that is growing, with no fever, is a reason to see a healthcare professional within 24 hours (Mayo Clinic). Cellulitis can spread rapidly, to the lymph nodes and the bloodstream if it isn't treated (American Academy of Dermatology), so don't let a growing rash go past a day.

Get care now if you have a fever with a bite that looks infected, or the swollen red area is changing fast (Children's Hospital Colorado, Mayo Clinic). With cellulitis, a very high temperature, feeling hot, cold, and shivery, a fast heartbeat or fast breathing, purple patches, dizziness, or confusion mean going to an emergency room (NHS).

A separate emergency is an allergic reaction: trouble breathing, swelling of the lips, face, eyelids, or throat, dizziness or fainting, hives, or vomiting. One or two of these is enough to call emergency services (Mayo Clinic).

Past that, naming the problem is the clinician's job. They'll ask when you were bitten and when the change started (Cleveland Clinic), then say whether it's cellulitis, impetigo, or a big reaction. Caught early, cellulitis usually clears completely without long-term problems (American Academy of Dermatology).

If you can't tell a normal reaction from an infection by looking, photograph the bite in the BiteSight iPhone app so you have a dated record to show a clinician. If you don't have an iPhone, the same photo tool works on the web. There's no Android app yet.