When two things are connected, you can say that there is "a link between" them. For example:

People realized a long time ago that there's a strong link between smoking and cancer. But a lot of people continue to smoke anyway.

When there's a link between two things, it usually means that one thing causes the other. But by calling "a link", you leave open which thing is the cause and which one is the effect.

You can use the verb "find" to talk about noticing or discovering links:

They were some of the first people to find a link between Vitamin D deficiency and breast cancer.