Transmission occurs rapidly via aerosolized respiratory droplets and contaminated litter or dust. The incubation period is extremely short (18–36 hours), with 100% of birds in an infected house contracting the virus within 48 hours.

Clinical manifestation depends on the specific IBV strain serotype (e.g., Massachusetts, 4/91, QX):

• Respiratory Form: Gasping, coughing, sneezing, and loud tracheal rales. Chicks huddle near heat sources. Trachea shows catarrhal exudate and congested mucosal lining.
• Nephropathogenic Form (QX / B1648 strains): Causes interstitial nephritis. Birds display wet droppings, severe thirst, and depression. Necropsy reveals swollen, pale kidneys with tubules and ureters engorged with white urate crystals.
• Reproductive Form (Layers): Drop in egg production (up to 50%). Eggs have wrinkled, rough, or thin shells and watery albumen ('runny egg white'). Infection of female chicks in the first 2 weeks of life causes permanent oviduct cyst formation ('false layer syndrome').

Diagnosis is confirmed via RT-PCR or ELISA. Due to frequent spike-protein gene mutations, dozens of IBV serotypes exist globally. Cross-protection between different serotypes is minimal (e.g., a Massachusetts vaccine will not fully protect against a QX strain outbreak).

Prophylaxis requires combining heterologous live strains (protect-type concept):

• Day 1 (Hatchery): Live IB Ma5 or H120 spray vaccination.
• Day 14: Booster with live IB 4/91 or QX strain via drinking water.
• Pre-Lay (Week 14–16): Inactivated oil-emulsion killed IB vaccine injection. Model schedules in the Vaccination Scheduler.