Although n-3 long-chain polyunsaturated fatty acids (n-3 LC-PUFAs) are used widely in the treatment of chronic inflammatory diseases, their effect in infectious disease requires a particular attention. The present article discusses their anti-inflammatory and immune properties involved in the host defence and presents a systematic review of the effects of their oral administration on the prevention and outcome of experimental and clinical infections. At a dose corresponding to an human dose of 500 mg/day, n-3 LC-PUFAs intake is beneficial against experimental infections caused by extracellular pathogens including Streptococcus pneumoniae, Pseudomonas aeruginosa, Escherichia coli, and Staphylococcus aureus by reducing inflammation, and reduces the incidence of pneumococcal infections in the elderly, but at 2-4-fold higher doses as occurs in some human intervention and/or during long-term it becomes detrimental in intestinal infections with Citrobacter rodentium or Helicobacter hepaticus by exacerbating anti-inflammatory response. They are also harmful against infections caused by intracellular pathogens as Mycobacterium tuberculosis, Salmonella, Influenza virus and Herpes simplex virus by affecting the immune cell response. The effects of n-3-LC-PUFAs on infections depend on the pathogen and the n-3 LC-PUFA dose and timing. Caution should be recommended for high-dose and long-term supplementation in humans.