The HHSRS describes a 'healthy indoor temperature' as being around 21°C. But one of the risk factors is people's perception: most of people won't 'feel' cold until the temperature drops below 18°C.
This is already 1°C below the level at which a person's immune system becomes comprised, creating a risk of that she or he will more easily catch a cold, flu or virus circulating in the community. Read more here.