Most databases allow you to type Boolean operators right into the search box. Each database is slightly different. Below are some examples from PubMed.
Dengue, malaria and Zika are mosquito-borne illnesses. Searching with AND between these terms returns only articles that mention dengue AND malaria AND Zika. There were 18 articles that referenced all three in the same article.
However, if we search with OR between these terms, we will get articles that reference dengue OR malaria OR Zika. Over 95,000 articles refer to one of these terms.
As you can see in the example above, Boolean operators (AND, OR and NOT) can be typed into the search box. Another way to conduct a separate search for each of your main concepts, then add them together in Advanced Search.