When you eat undercooked cod or other undercooked fish you put yourself at risk for getting infected with bacteria that the cod or other fish could be infected with.
When you eat undercooked cod or shellfish you then increase your risk of exposure to Vibrio bacteria.
The Vibrio bacteria can make you ill and can lead to diarrhea, vomiting, stomach pain and nausea and in some rare cases death can occur when eating undercooked cod due to the infection of bacteria.
I would not want to risk it and would make sure that I had cooked my fish and cod fully before eating it because getting sick is not fun and it's not worth the risk of possibly dying as a result.
Some people have eaten undercooked cod without any health issues but for me it's not worth risking my health over.