What are common causes of fever in infants under three months of age?

In infants under three months of age, fever is often a sign of an underlying infection due to their still-developing immune systems. Common causes of fever in this age group include viral infections and urinary tract infections.

Viral infections can present as a fever because they trigger the body's immune response, leading to an increase in body temperature as the body fights off the virus. Conditions like respiratory syncytial virus (RSV) and other viral illnesses can be common in this age group.

Urinary tract infections (UTIs) are also a notable cause of fever in infants, as they can occur even in very young children and often manifest primarily as a fever without other clear symptoms such as vomiting or diarrhea.

While other possibilities, like bacterial infections, do exist, the specific mention of the combination of viral infections and UTIs aligns with clinical observations and guidelines that emphasize the need for careful evaluation of febrile infants for these two common causes.