Some species are used to treat and prevent common cold, flu, and other infections.
One study from 2002 didn't find any significant differences between the echinacea and placebo groups for any measured outcomes. Another study from 2003 confirmed that Echinacea purpurea was not effective in treating URI (upper respiratory tract infections) symptoms in children and its use was usually associated with an increased risk of rash. The more recent one from 2014 didn't provide enough benefits for treating colds by using Echinacea products, although there could be a weak benefit, but potential effects are of questionable clinical relevance.
So two NCCIH-funded studies didn't find any benefits from using it. Others found that it may be beneficial, so the results are mixed and it's not clear whether it can prevent or effectively treat URIs (such as common cold) and NCCIH is continuing to support the study of echinacea as well as potential effects on the immune systemNCCIH.