The most common signs of an infection are fever, pain, swelling, redness, and warmth at the affected area. These show up whether the infection is in a small cut on your hand or deep in your urinary tract, though the specific combination varies depending on where in your body the infection takes hold. Some signs are visible on the surface, while others are internal signals your body sends to tell you something is wrong.
The Five Classic Signs of Inflammation
When your body detects an invading pathogen, it launches an inflammatory response at the site of infection. This produces five hallmark signs that have been recognized since ancient medicine: redness, swelling, heat, pain, and loss of function. Each one has a specific biological cause. Redness and heat come from increased blood flow to the area as your immune system rushes cells to fight the infection. Swelling happens because blood vessels become more permeable, leaking proteins and fluid into the surrounding tissue. Pain results from chemical signals irritating nearby nerve endings, and the combined effect of all four often makes it difficult or impossible to use the affected body part normally.
These signs are easiest to spot when the infection is near the skin’s surface, like an infected wound, an insect bite that’s gone bad, or a swollen, red throat. But the same process happens internally. A kidney infection produces pain and swelling you can feel in your lower back or side. A bladder infection causes burning pain when you urinate. The inflammation is identical; you just can’t see it directly.
Whole-Body Symptoms
Fever, fatigue, muscle aches, and headache are typically the first evidence that your body is reacting to a pathogen. These aren’t caused by the infection itself but by your immune system’s chemical alarm signals. Proteins released by immune cells travel to the brain and trigger the hypothalamus to raise your body temperature, producing fever. Those same chemicals cause the widespread achiness and exhaustion that make you want to stay in bed.
A body temperature between 100.4°F and 102.2°F is generally considered a low-grade fever. When it climbs above 102.2°F, that can indicate a more serious infection. Chills often accompany the rise in temperature as your body works to generate heat, and you may notice alternating waves of feeling cold and then hot.
Not every infection produces a fever, though. Some low-level infections stay localized without triggering much of a systemic response, while certain people (particularly older adults) may fight serious infections without developing a significant fever at all.
Signs of a Wound Infection
If you have a cut, scrape, surgical incision, or any break in the skin, watch for changes in the days following the injury. Increasing redness that spreads outward from the wound edges, growing warmth, and worsening pain are early indicators. Swelling that gets worse rather than better is another red flag.
Drainage from the wound is one of the clearest signs. Infected wounds often produce pus that can be white, yellow, green, pink, or brown, and it usually smells bad. If the color or odor of the drainage changes, that typically means the infection is worsening. Red streaks extending away from a wound along the skin suggest the infection is spreading into the lymphatic system, which needs prompt attention.
Swollen Lymph Nodes
Your lymph nodes act as filtering stations for your immune system, and they swell when they’re actively fighting an infection. The location of the swelling often points directly to where the infection is. Swollen nodes in your neck commonly accompany throat infections or upper respiratory infections. Swollen nodes in your groin may signal an infection in your leg or pelvic area. Nodes in your armpit can swell in response to an infection in your arm or chest.
Swollen lymph nodes from a routine infection are typically tender to the touch, movable under the skin, and shrink back to normal as the infection clears. Nodes that are hard, painless, or continue to grow over weeks warrant a closer look from a healthcare provider, as those characteristics are less typical of simple infections.
Viral vs. Bacterial Infection Signs
Viral and bacterial infections can look very similar at the start, but a few patterns help distinguish them. A common viral upper respiratory infection usually brings a runny nose, cough, low-grade fever, sore throat, and trouble sleeping. In children, these symptoms can last up to 14 days and happen six to eight times per year on average.
A bacterial infection is more likely when symptoms persist beyond the 10 to 14 days a virus typically lasts, when the fever is higher than you’d expect, or when the fever gets worse several days into the illness rather than improving. Sometimes bacteria move in after a virus has already weakened your defenses. A runny nose that lingers past two weeks, for example, may have started as a viral cold but developed into a bacterial sinus infection.
Signs of Internal Infections
Infections inside the body don’t always produce visible signs on the skin, so you have to rely on how you feel and where the pain is located. A bladder infection commonly causes pain or burning during urination, a frequent urgent need to urinate, and sometimes cloudy or strong-smelling urine. If the infection moves up to the kidneys, you may develop lower back pain or pain along the side of your back, along with fever and nausea.
Respiratory infections often announce themselves with a persistent cough, chest tightness, shortness of breath, or thick mucus production. Abdominal infections may cause localized pain, nausea, vomiting, or diarrhea. The key pattern with internal infections is that symptoms tend to be concentrated in one region of the body and worsen over time rather than gradually improving.
Infection Signs in Infants
Babies can’t describe their symptoms, so parents need to watch for behavioral changes. A baby who is feeding poorly, sleeping far more than usual, or showing poor skin color may be getting sick. Any fever in a baby under one month old is treated seriously, even if the baby otherwise looks well.
Dehydration is a particular concern with infant infections. If your baby has not urinated in eight hours, that suggests significant fluid loss. A sunken soft spot on the top of the head also signals dehydration. Conversely, a soft spot that is tense and bulging can indicate pressure on the brain, which is a medical emergency regardless of cause.
Warning Signs That an Infection Is Serious
Most infections resolve on their own or with straightforward treatment, but some progress to a dangerous level. The signs that an infection is becoming systemic and potentially life-threatening include rapid breathing (22 breaths per minute or more in adults), a noticeable drop in blood pressure that causes dizziness or lightheadedness, and confusion or altered mental state. These three indicators together suggest the body is struggling to maintain normal organ function under the stress of infection.
Other serious warning signs include a fever above 103°F that doesn’t respond to fever-reducing measures, a rash that spreads rapidly, difficulty breathing, severe or worsening pain, and an inability to keep fluids down. In someone who already has a known infection, any sudden worsening of symptoms or a new wave of fever after initial improvement can signal that the infection has spread or that a secondary bacterial infection has developed on top of an initial viral one.