Perform simple searches, like the ones you use in Google. A single term for a condition and another for an intervention may suffice.
...vitamin C cold
There is no real need of using a taxonomy, or boolean terms, such as AND or OR (they work, but are rarely needed. If you are really fond of this kind of search strategies, you can use our advanced search).
You can search in your own language or in English. You can even combine terms from different languages.
Echinagard treatment shortens the course of the common cold: a double-blind placebo-controlled clinical trial