Cough, fever, a runny nose, or feeding difficulties may be signs of an illness. Knowledge of the most common illnesses in infancy allows parents to recognise worrying symptoms more quickly and respond appropriately. Although many infections are mild and do not require antibiotic treatment, there are situations in which an urgent consultation with a paediatrician is necessary.
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In this article, we discuss the most common conditions affecting infants, both viral and bacterial infections as well as typical issues associated with infancy. We also explain when it is worth consulting a doctor and how to support your baby’s immunity in everyday life.
What are the main features of an infant’s immune system?
Immune system of a newborn and infant differs from that of older children and adults. In the first months of life body is only beginning to learn how to recognise and respond to pathogens. That’s why infants are more vulnerable to infections, and their course may be atypical as fever or cough does not always occur.
Initial protection is provided by mother’s antibodies, especially if the baby is breastfed. Breast milk contains immunoglobulins (mainly IgA), which support protection of mucous membranes, as well as natural prebiotics and immune cells. Breastfeeding is therefore one of the most important ways to support an infant’s immunity in the first six months of life.
Child’s immunity develops gradually, both through contact with the natural environment and through protective vaccinations. During the first year of life immune system matures, learning to recognise viruses, bacteria, and other pathogens. This is a natural process and may be associated with frequent, but usually mild, infections.
It is worth remembering that colds and other illnesses in infancy are, in most cases, a physiological part of immune system development and do not necessarily indicate poor health. However, observing symptoms carefully and responding appropriately when warning signs appear is crucial.
Viral infections – the most common cause of fever
Viruses are the most common cause of illnesses in infancy. Due to an immature immune system, even a common cold can trigger a strong reaction in a baby, including high fever, irritability, and feeding or sleep problems. Symptoms often develop suddenly, and their severity may change within just a few hours.
One of the most common infections is the common cold (upper respiratory tract infection). It presents with a runny nose, cough, irritability, and a raised temperature. Among infants, the RSV virus (respiratory syncytial virus) can be particularly dangerous, as it may cause bronchiolitis and breathing difficulties.
Other viral causes of fever include:
- Roseola (exanthem subitum) – a typical illness of infancy, usually occurring between 6 and 24 months of age. It is characterised by a sudden high fever lasting 2–3 days, followed by a pale pink rash.
- Rotavirus infections – cause diarrhoea, vomiting, and fever. They may lead to rapid dehydration and therefore require close monitoring.
- Influenza – although less common in very young children, it may have a more severe course, with high fever, weakness, and loss of appetite.
In viral infections, treatment is usually symptomatic: hydration, rest, fever reduction, and medical consultation if needed. Most viral infections in infants resolve on their own, but even a seemingly mild illness may require a paediatric consultation if symptoms are severe or prolonged.
Bacterial infections – when antibiotics are needed
Although most illnesses in infancy are viral, bacteria can sometimes be the cause. In such cases, antibiotic therapy may be necessary, as bacterial infections do not resolve on their own and, if left untreated, may lead to complications.
One of the most common bacterial infections in infants is middle ear infection (otitis media). It may occur as a complication of a cold or as a standalone infection. Symptoms can be non-specific: irritability, crying during feeding, pulling at the ear, and fever. In most cases, a medical consultation is necessary so the doctor can assess the eardrum and decide on treatment.
Another serious infection is pneumonia, which in infants often has an atypical presentation. Instead of classic cough or shortness of breath, symptoms may include sleepiness, apathy, feeding difficulties, and fever. Due to the risk of complications, any suspicion of pneumonia requires prompt diagnosis and, if necessary, antibiotic treatment.
Urinary tract infections (UTIs) are also relatively common in infants. In very young children, they may present only with fever and no other obvious symptoms. For this reason, any unexplained fever in a child under 3 months of age should be thoroughly investigated, including for a possible UTI.
A rarer but possible bacterial condition is streptococcal tonsillitis. It mainly affects older children, but it cannot be completely ruled out in infants. It is characterised by high fever, sore throat, and difficulty swallowing.
In cases of recurrent throat infections or persistent discomfort, you may also want to read the article: Paediatric ENT specialist – looking after healthy child’s throat.
Unlike viral infections, bacterial illnesses usually require antibiotics. Their use should always be justified - based on symptoms, test results, and a medical diagnosis.
Other common infant health problems
Not all conditions occurring in the first year of life are caused by infections. Some symptoms that worry parents are related to physiological developmental processes, reactions to new foods, or gastrointestinal sensitivity. Although these are not diseases, they often cause discomfort and may require paediatric support.
One of the most common problems in the first months of life is infant colic. It presents as sudden, difficult to soothe crying, usually occurring in the evening. Baby may draw up their legs, tense their abdomen, and show signs of pain. Colic is not dangerous, but it can be very challenging for both the child and parents. The exact causes are not fully understood and may be related to gastrointestinal immaturity, gas accumulation, or gut microbiota.
Another common issue is gastro-oesophageal reflux. In most infants, it is physiological and presents as regurgitation after feeding. In some cases, however, reflux may lead to irritability, poor weight gain, or even oesophagitis when a medical consultation is required.
Teething is a natural developmental stage that often comes with symptoms resembling infection, such as a slightly raised temperature, irritability, drooling, and reluctance to eat. Some children go through this stage mildly, while others experience noticeable behavioural changes.
Food and skin allergies are also increasingly diagnosed in infants. The most common symptoms include rashes, diarrhoea, spitting out food, bloating, and skin problems such as atopic dermatitis. In such cases, observing the child’s reactions to specific foods (especially cow’s milk, eggs, and gluten) is important, and consultation with an allergist or dietitian may be necessary.
Food allergies and skin reactions, digestive issues, or bedwetting are problems that often worry parents. If you would like to learn when bedwetting becomes a reason for medical consultation, see the article: Bedwetting – when should you be concerned about your child?
When should you see a doctor about your infant?
Diagnosing illness in an infant can be challenging, as babies cannot express pain or discomfort. That is why it is so important for parents and caregivers to recognise alarm symptoms that require urgent paediatric consultation. A quick response can prevent complications and shorten treatment time.
You should seek medical attention immediately if your child develops:
- a fever above 38°C in an infant under 3 months of age,
- breathing difficulties, wheezing, or rapid chest movements,
- marked lethargy, drowsiness, or lack of eye contact,
- feeding problems – refusal to eat, difficulty swallowing, vomiting,
- signs of dehydration – dry nappies for several hours, dry tongue, sunken fontanelle,
- seizures, sudden limb jerking, or a sudden loss of muscle tone,
- a rash that does not fade when pressed (which may indicate a serious infection),
- persistent, inconsolable crying lasting many hours without an obvious cause.
It is also worth consulting a paediatrician if symptoms persist for more than a few days, recur, or if the child becomes ill more often than peers. Even a seemingly mild infection, if prolonged, may require a change in treatment or additional tests.
In case of any doubt, it is better to act sooner rather than later. Observing your child’s behaviour and knowing the warning signs are key to prompt action and effective help.
The most common illnesses in infancy – summary
Illnesses in infancy are an inevitable part of early childhood. Although most have a mild course and do not require complex treatment, some situations may require prompt medical intervention. Knowing typical symptoms, observing your child closely, and understanding when to consult a paediatrician help minimise the risk of complications and support their healthy development.
