Many studies have shown that BCG is effective in reducing deaths from TB disease, particularly in children. However, there is also evidence that in countries with a relatively high death rate, BCG reduces this death rate but by more than would be expected by its effect on TB alone. This controversial but important finding warrants further and serious consideration. One possibility is that BCG, in addition to stimulating the immune system against TB, also has a positive effect on the way the immune system works more generally. This might mean a more effective response to serious infections that are completely unrelated to TB, or the ability to respond better to other routine childhood vaccines, or to maintain such responses for longer. In these examples, the timing of BCG administration in relation to other vaccines could be crucial.