How would you react if you found yourself attracted to a client? How might you respond to a client who reveals an attraction to you?

This is an extremely difficult predicament. First and foremost, if I found myself attracted to a client, or a counseling patient in particular, I would sever the relationship as best as possible. The first step would be to find a practitioner who would be able to take them on as a client that you trust and that you know would give them the best treatment possible. But it is imperative that you extricate yourself from the situation on the grounds of professionalism. In the field of mental health in particular, it can be very complex, because the client is very open and vulnerable with their counselor, often discussing relationships and romantic intentions. This can intermingle the professional relationship with a more intimate one, and there must be no interconnection that could give cause for concern, both for yourself professionally and for their own mental health. Your duty as a counselor is to provide a listening ear and potentially guidance for their improvement and support. If you cannot give unbiased support, you are not acting in their best interests.
If the opposite situation occurred, where a client revealed an attraction to myself, I would have to take similar steps. It would do a disservice to them to continue working with them when A) I am not attracted to them and B) it is against my professional code and standards to pursue any sort of relationship with them. I would take much the same steps, finding them a competent provider who could help them with their needs. If some of the attraction stems from some sort of mental instability (such as repressed emotions toward someone else or a clinical dependency syndrome), I may continue some engagement with the client, but in a more controlled setting, preferably with another individual present to act as a chaperone and mediator, to prevent any issues arising. (There is always the potential of rumors or lawsuit if a client makes romantic advances toward you, so it is best to have witnesses and corroboration of your meetings to ensure that you are not held liable for false accusations.)
The main point is that your duty is always to act in the best interests of the client and that it is extremely important to maintain professionalism and to keep every action you take ethical and above reproach.