Ferrum phos is unusually prescribed on the basis of a lack of symptoms. Strange as this may sound people needing this remedy may well have a high fever and no other symptoms!
Even if they have a very high temperature there well be no other marked symptoms that would guide you a different remedy.
They may well be thirsty for cold drinks and have a flushed facial appearance.
Strangely, an absence of clear, qualifying symptoms is a key symptom
Colds and Flu
Fever