Abstract The term ‘malnutrition' usually refers to macronutrient malnutrition, though individuals may also be malnourished in terms of micronutrients. Worldwide, it is commonly associated with poverty. Malnutrition is a risk factor that increases susceptibility to and severity of infections by its effects on both innate and adaptive immunity. It is also common in hospitalized patients, where it can be readily assessed at the bedside using simple tools. Blood tests have little value in assessing nutritional status in the context of infection, because of the influence of the acute-phase response. Conversely, both acute and chronic infections impair nutritional status. Although reduced intake appears to be the prime driver for weight loss in infective states, infection may reduce the ability of the body to utilize nutrients. This is particularly the case in severe sepsis, in which there may be a relatively narrow therapeutic window between adequate and excessive nutrient supply. Nutrition support follows a hierarchical approach, starting with dietary counselling, progressing to use of supplements and then to enteral nutritional support with nasogastric or percutaneous endoscopic gastrostomy tube-feeding for short-term and longer-term feeding respectively; total parenteral nutrition is seldom required.