People with the best social networks, who sing in choirs; play instruments; go to church and take part in team sports, all live longer and tend to be happier, studies have shown. People who feel lonely, or isolated, on the other hand, fare less well and are more prone to ill-health. Now a new study, published this week in the journal PNAS and examining the immune systems of lonely people and monkeys has revealed why this might be. John Capitanio revealed all to Chris Smith...