Recent outbreaks and spread of tilapia lake virus (TiLV) in several countries were reported. It appears that tilapia at all age groups is susceptible to TiLV infection. However, there is limited understanding how TiLV produce disease in tilapia. Laboratory challenge of naïve Nile tilapia (Oreochromis niloticus) and red hybrid tilapia (Oreochromis spp.) develop clinical signs of TiLV infection including skin redness, skin erosion, anemia, exophthalmos, corneal opacity, and abdominal swelling. The histopathological lesions of challenged fish included syncytial formation and necrosis of liver cells with eosinophilic intracytoplasmic inclusion body. In the hematopoietic tissues such as spleen and anterior kidney of challenged fish, depletion of red blood cells and infiltration of lymphocytes with foamy appearance cells are commonly observed in heavy TiLV-infected fish. To date, the TiLV disease could be produce via intraperitoneal injection or cohabitation of susceptible fish with disease fish. The cohabitation model suggests that TiLV is horizontally transmitted in the fish population via direct contact of moribund fish. To support this hypothesis, we demonstrated that TiLV genomic RNA could be detected in mucus using reverse transcription quantitative polymerase chain reaction (RT-qPCR). Moreover, mucus from TiLV-infected fish produces CPE formation in E-11 cells suggesting a live viral particles are present in the mucus. Commonly, TiLV and various bacteria could be detected in moribund fish. Our results showed that concurrent infection of fish challenged with TiLV and bacteria produce severe clinical signs with higher mortality than the single infection. Severe inflammation and tissue damage were obviously notified in the co-infected fish. Currently, there is little information available how TiLV cause disease in tilapia. Recently, our study revealed that most important warm water fish species are resistant to TiLV infection with exception of giant gourami (Osphronemus goramy). Genetic analysis of TiLV from different geographic isolates revealed the virus is divergent. Although more information improve our knowledge of TiLV pathogenesis, additional experiments will help us understand how TiLV cause infection and induces severe diseases in tilapia. These data could be applied for the intervention measures to control the disease.