My bug bite is still spreading. When should I worry?

A bite or sting puts something foreign into your skin, and your body reacts to it. When a mosquito feeds, it injects saliva, and the bump and the itch are your body responding to that saliva (CDC). Some people react much more strongly, with a large area of swelling, soreness, and redness. The big version has a name, skeeter syndrome, and it's most common in young children (Cleveland Clinic).

That kind of reaction usually starts 8 to 10 hours after the bite and settles within 3 to 10 days (Cleveland Clinic), so a bite that looked small at bedtime and bigger at breakfast fits. For ordinary insect bites in children, pinkness usually lasts about 3 days and swelling up to 7, and redness alone doesn't mean the bite is infected (Children's Hospital Colorado).

Stings keep growing for a while too. A moderate reaction to a bee, wasp, or hornet sting brings swelling that gets worse over the next day or two and can last up to seven days (Mayo Clinic). A large local reaction is swelling 10 cm (about 4 inches) or more across, and it typically grows for one to two days before clearing over three to ten (Canadian Paediatric Society). People say a wasp bit them, but it was a sting, and these are the numbers that go with it.

The infection people worry about is cellulitis. Bacteria get in through a break in the skin, and a bite or a scratch can be that break, sometimes too small to see (NHS). The skin turns swollen, painful, and warm, and fever or chills can follow (Mayo Clinic). A big reaction to saliva can also look red and warm, so color and warmth alone don't settle it.

What separates them is what the area does next. A reaction grows for a day or two, then stops and slowly clears. Cellulitis redness keeps getting bigger as the infection spreads, and a sore or rash that starts suddenly can grow quickly in the first 24 hours (MedlinePlus).

Spread also has a shape. A red streak running out from the bite, instead of a patch around it, is a sign of an infected mosquito bite (CDC), and a streak that keeps spreading is a reason to contact a provider right away (MedlinePlus).

The clearest timed rules for ordinary insect bites come from Children's Hospital Colorado. Contact a doctor within 24 hours if new redness shows up more than 48 hours after the bite, if it's been more than 72 hours and the redness is still getting larger, or if the red area or a red streak grows past 1 inch (2.5 cm) (Children's Hospital Colorado). That guide is written for children and leaves out bees, mosquitoes, fire ants, ticks, and spiders, so treat it as a yardstick, not a deadline for every bite. Whether it fits you is a question for a clinician, and any one of those is a fair reason to call.

The cutoffs are short because untreated cellulitis can spread fast, to the lymph nodes and then the bloodstream (American Academy of Dermatology). Swelling that gets worse and not feeling well belong on the same list (Mayo Clinic). With cellulitis, not feeling well shows up as flu-like symptoms and swollen, painful glands (NHS).

The doctor triage tool runs through the same questions in a minute, including spreading redness and fever, and sorts the answer into home, call a clinician, or emergency care.

The sources don't use the phrase urgent care, but they sort a growing red area into three speeds. Skin that is painful, hot, and swollen is an urgent call: in the UK that means an urgent GP appointment or NHS 111 (NHS). A swollen rash that is growing without a fever is a visit within 24 hours. A rash that is changing rapidly, or any fever with it, is emergency care (Mayo Clinic). A clinic that sees people the same day fits the first two speeds, and the emergency room is for the third.

The NHS also lists the signs of a serious spread: a very high temperature or feeling hot, cold, and shivery, a fast heartbeat or fast breathing, purple patches on the skin, dizziness or fainting, confusion, or cold, clammy, pale skin. Whether that fits you is a question for a clinician, and if any of those show up with a spreading red area, get care now. A separate list covers allergic reactions. Trouble breathing, swelling of the lips, face, eyelids, or throat, fainting, a weak and rapid pulse, hives, or nausea, vomiting, or diarrhea call for emergency help even if only one or two are present (Mayo Clinic).

The two things a clinician most wants are when you were bitten and when the redness started. Doctors ask exactly that about a large mosquito reaction, along with recent travel, fever, and trouble breathing, because no test settles it (Cleveland Clinic). Write down the time of the bite if you know it, when you first noticed the red area, and anything new since, like a fever or a streak.

How fast it grew is easier to show than to remember. A provider may draw around the edge of the redness with a pen and check over the next several days whether it crosses the line (MedlinePlus), and UK guidance for clinicians suggests the same (NICE). If you're waiting a few hours or a day for an appointment, you can draw a thin dated line along the edge yourself, or take a dated photo from the same spot in the same light. The next look then shows whether the edge moved.

Where the red area is changes what it points to. Cellulitis most often involves a lower leg and usually just one side of the body (Mayo Clinic). In children it tends to show up on the face or neck, and redness and swelling in both legs at once usually means another condition, such as stasis dermatitis or contact dermatitis (American Academy of Dermatology). One bite on one shin is a weak explanation for two red legs, and a clinician is the one to sort that out.

Around an eye the answer splits. After a bite on the upper face, swelling by the eye looks dramatic but is harmless for the bites the Colorado guide covers, and it's worse in the morning and easier after a few hours upright (Children's Hospital Colorado). Cellulitis in or around the eye is very serious (NHS). Eye swelling that is hot, painful, or still growing after the child has been up looks more like the second picture than the first, and is a reason to be seen.

A baby lowers the bar. The NHS says a child under 1 with an insect bite or sting should see a GP (NHS), so don't spend a day measuring the edge first.

The red patch of early Lyme disease doesn't appear with the bite. It starts at the bite site 3 to 30 days later, about 7 days on average, and grows gradually over several days, up to 12 inches (30 cm) or more across. It may feel warm, is rarely itchy or painful, and doesn't always look like a bullseye (CDC). A small red bump right where the tick was, one that looks like a mosquito bite, is common and fades in a day or two. Redness that shows up days after a tick and keeps growing is a reason to call a clinician and mention the tick, and the same goes if you live where Lyme occurs or traveled to such a place. In the UK, the NHS says to see a GP for a tick bite with flu-like symptoms or a round or oval rash (NHS). The tick bite with no bullseye guide goes further.

If it's still growing after the second day, hurts more, or comes with a fever, call a clinician. If you simply can't tell whether the edge moved, photograph the bite in the BiteSight iPhone app, or use the same photo tool on the web if you don't have an iPhone. There's no Android app yet.