What do you suspect in a patient with a headache, cough, fever, and eosinophilia but negative blood cultures?

The combination of headache, cough, fever, and eosinophilia raises suspicion for coccidioidomycosis, also known as valley fever. This fungal infection is endemic in certain areas, particularly in the southwestern United States. The symptoms presented align well with an uncomplicated respiratory form of this disease, which can indeed include headache and cough, in addition to fever.

Eosinophilia is particularly notable in this context, as it can occur in response to certain fungal infections, including coccidioidomycosis. The negative blood cultures further support this diagnosis, as coccidioidomycosis is not typically detected through standard blood culture methods that are used for bacterial infections.

While pulmonary tuberculosis and lymphoma can also present with similar symptoms, they are less likely to be associated with eosinophilia in this context, especially with negative blood cultures. Asthma, while it can also cause cough and possibly a fever in acute exacerbations, would not typically present with eosinophilia to such an extent and is less likely to explain the combination of symptoms in a new onset scenario. Thus, coccidioidomycosis becomes the most plausible diagnosis given these clinical findings.