AVN is an acronym for Acute Viral Necrosis, a form of viral infection caused by the Herpes Simplex Virus (HSV). It is most commonly seen in fish, but can also affect other animals, including birds and reptiles. The 4 stages of AVN include: Stage 1: Latent Infection; Stage 2: Clinical Signs; Stage 3: Damage to Organs; and Stage 4: Death. During the latent infection stage, the virus is present in the body, but does not yet cause any clinical symptoms. During the clinical signs stage, the virus begins to cause symptoms such as reduced appetite, lethargy, and difficulty breathing. During the damage to organs stage, the virus can cause damage to the organs and tissues, resulting in poor growth, organ failure, and even death. Finally, during the death stage, the virus has caused so much damage that the animal dies.