One of the main reasons fridge temperatures are recommend to be set at 5ºC is because of listeria. Listeria is a food poisoning bacterium that is psychrotrophic – in other words it likes cold temperatures. From the growth models, we know that listeria grows nearly twice as fast at 8°C as it does at 5°C, hence the recommended 5ºC, which helps keep us safe from listeria.
Source: wiktionary