Vitamin C

Remember the House episode where Scurvy was mistaken for Small Pox? The first scene was a disease breakout aboard a ship in the 17th century. That led to the discovery of vitamin c. The extreme deficiancy resulted in what we know today as Scurvy.

Now, on to the really interesting stuff.

Echinacea

Functions
This beautiful cone flower is often mentioned together with the immune system (the link is to a wiki, but I should write a easy-to-understand article soon). Do you suffer from the common cold? This herb has the ability to boost your immune system during cold seasons to help your body naturally fight colds and infections.

Consumption
Echinacea can be consumed in the form of pills or as a tea.

Doses
So how often do you use it? From what I have read so far, it is only used whenever you want to boost your immune system. My inference is that it should not be part of the regular supplements routine.

Personally, I would depend on vitamins and minerals from consumption of daily foods. This promotes overall health which will include the body's ability to fight viruses and infections.

Precautions
Echinacea causes strain on the liver. Do not use continuously for more than 3 months.

Further References
http://www.articlesbase.com/supplements-and-vitamins-articles/echinacea-health-benefits-3374601.html