The effects of gaffkaemia infection include lethargy (typically seen as a drooping tail), anorexia and a pink colour on the ventral side of the abdomen, which gives the disease its alternative common name of red tail disease. When lobsters are moribund, they may lie on their sides, and frequently lose appendages. The effects of gaffkaemia are slowed by low temperatures, such that death can occur within two days of infection at , but can take over 60 days at .

As few as five bacteria can lead to clinical disease. When they enter the host, the bacteria colonise the heart and hepatopancreas. They may be engulfed by phagocytosis into the lobster's blood cells, but continue to survive within the blood cells, feeding on the cytoplasm. The lobster's blood cell count drops, and the infection develops into septicaemia. The stores of glycogen in the hepatopancreas become depleted, concentrations of glucose and lactic acid in the blood drop, and concentrations of adenosine triphosphate in muscles also fall. In a severe infection, the ability of the lobster's blood pigment haemocyanin to carry oxygen may be reduced by up to 50%.

Gaffkaemia (gaffkemia in American English) is a bacterial disease of lobsters, caused by the Gram-positive lactic acid bacterium Aerococcus viridans" var. "homari.