Signs of FIP can arise weeks, months, or even years after initial infection. Early on, a cat may be asymptomatic or show vague signs:
Stunted growth / growth retardation.
Increased susceptibility to common infections.

Note: Early upper respiratory signs are usually caused by secondary pathogens (herpesvirus, mycoplasma, etc.), not directly by the FIP virus itself.

When FIP transitions into the clinical stage, cats often show general symptoms that are resistant to standard antibiotics:
Fluctuating, antibiotic-unresponsive fever.
Loss of appetite (Anorexia) & weight loss.
Lethargy and depression.
Rough, unkempt coat.