Hi, i've been having fever since last week on saturday with a mere small headache sometimes, what could be the issue?

Fever & FUO: Fever can result from many conditions. It is a very common problem in clinical practice. Infections (i.e. COVID-19, Flu, pneumonia, pharyngitis, abscesses, skin infections, urinary tract infections, etc), connective tissue diseases (i.e. vasculitis, systemic lupus erythematosus, etc), and malignancies (cancers, i.e. liver cancer, etc) classically explain a febrile illness without an obvious cause