Persistent fever in a child — when is it more than an infection?

When the body produces too few healthy red blood cells — a condition called anaemia — children look noticeably pale, sometimes almost grey or waxy. This is different from a child who is simply fair-skinned. Look at the inner eyelids, gums, and nail beds: if they have lost their usual pink colour, that is a more reliable sign of anaemia than skin tone alone. Anaemia in children is most often due to diet or a viral illness, but in leukemia it occurs because abnormal white blood cells crowd the bone marrow, leaving too little space for red blood cells to form. Paleness that develops over a few weeks and does not improve with iron-rich food or standard supplements deserves a blood test.

Children fall and bruise — that is completely normal. The concern is when bruises appear in unusual places (the back, upper arms, torso) without any memory of a knock, or when they are larger and darker than expected for the impact. Equally telling are petechiae — flat, pinpoint red or purple spots that look like a rash but do not blanch when you press a glass against them. Petechiae appear when platelets, which help blood clot, are very low. A low platelet count (thrombocytopenia) alongside persistent fever is a combination that paediatric oncologists take seriously, because it can indicate that the bone marrow is not producing normal blood cells.

Children with leukemia sometimes complain of deep, aching pain in their legs, arms, or back — pain that is hard to localise and that is often worse at night. This bone pain happens because the expanding mass of leukemic cells puts pressure on the bone marrow from inside. Young children who cannot describe pain may simply stop wanting to walk, refuse to bear weight, or become unusually clingy and irritable in the evenings. These complaints are easily mistaken for growing pains. The distinguishing feature is that true growing pains are symmetric, brief, and happen in the limbs — not in the back or joints — and they improve with massage. Pain that wakes a child repeatedly from sleep and is associated with fever warrants medical review, not reassurance alone.

Lymph nodes swell during almost every childhood infection — that is how the immune system works, and a pea-sized, tender lump in the neck after a throat infection is completely expected. Concerning nodes are different: they are larger (roughly more than 1 cm across), firm rather than soft, painless or only mildly tender, and they do not shrink within two to four weeks of the infection clearing. Nodes that are fixed to surrounding tissue rather than freely moving, or that appear in multiple regions at the same time alongside a prolonged fever and night sweats, raise the possibility of lymphoma — a cancer of the lymphatic system that responds well to treatment when caught early.

Every child sweats at night sometimes, especially in warm weather. The sign that concerns paediatric oncologists is drenching night sweats — sweating that soaks the child's pyjamas and bed sheets, occurring regularly over several weeks, in a room that is not unusually warm. Together with fever and unexplained weight loss, drenching night sweats form what oncologists call the "B symptoms" associated with Hodgkin lymphoma and some non-Hodgkin lymphomas. These symptoms alone do not confirm lymphoma, but their combination over two weeks or more is a reason to proceed with blood tests and imaging promptly rather than watching and waiting further.

Children's appetites naturally fluctuate, and a few days of poor eating during any illness is normal. The concern arises when a child is noticeably losing weight over several weeks — clothes become loose, the child looks thinner in the face and limbs — without any deliberate change in diet or a clear illness to explain it. Cancer cells consume energy and release substances that suppress the appetite. This happens across multiple cancer types, including leukemia, lymphoma, and solid tumours like neuroblastoma. If your child has lost weight alongside a persistent fever and one or more of the other signs listed here, that combination is the signal to arrange a paediatric review within days, not weeks.