All Jews are responsible for one another (Shevuos 39a).
The commentators explain the full extent of our responsibility for one another: If any Jew has been derelict in performance of a given commandment, every other Jew is considered to be derelict in that particular mitzvah, even though he or she may have performed it to one's fullest capacity. All Jews are considered to be a single unit. Just as the unit is incomplete if any part of it is missing or broken, so too, no one can consider oneself complete if any other part of the "unit" is incomplete.
A person who sustained an injury causing infection to one arm would not say, "It is only my arm that has been injured; therefore, no other part of my body has been affected." Since the body is a unit, anything that affects the part affects the whole.
People are physically distinct, and their spirituality is an intangible entity; that is why we do not readily perceive the spiritual forces that unite us. Nevertheless they are very real. However, just as it is possible for part of the body to be anesthetized so that experiences no sensation from what transpires in other parts of the body, so it is possible for there to be a "spiritual anesthesia" which renders us insensitive to the spiritual injury that may occur anywhere within the body of universal Jewry. We must overcome this insensitivity if we are to be a healthy and optimally functioning nation. We must learn the vital lesson that we are enhanced by the spiritual successes of our neighbors, and we are diminished by their failures.
To the question, "Am I my brother's keeper?", the answer is, "Yes!"