The document discusses infectious coryza, caused by Haemophilus paragallinarum bacteria. There are two main serotyping schemes for H. paragallinarum - Page and Kume. Page recognizes 3 serovars (A, B, C) that represent distinct immunovars and vaccines for one serovar do not protect against the others within the same serovar. Kume also recognizes 3 serogroups that correspond to Page serovars. Chickens develop varying degrees of natural immunity after infection, with full homologous immunity developing within 2 weeks against the same serovar but also some cross-protection between Page serovars.