EBV (the Epstein-Barr virus) seems to appear among clusters of people from time to time, with symptoms similar to those you describe (especially if there's also extreme tiredness). So glandular fever (which is what EBV causes in in adults, because its effects can go almost unnoticed in children) is one possibility. It's becoming more common these days because, due to higher hygiene standards, fewer children contract it (when it would hardly harm them anyway), leaving them without any immunity as adults (when it knocks them for six).