While in a perfect world every caregiver would have extensive medical knowledge, it is not a realistic expectation in practice. Because of the swiftness that illness can come, many times a caregiver is put into action without ever considering the role as a possibility. Sometimes a caregiver does not volunteer for the duty, they are drafted into the position at the request of a loved one in need, and out of love and respect agree to the task. In most situations of a caregiver providing medical assistance at home, there is a support system in place consisting of family that can ease the load and give valuable input to create a sort of "care by committee" atmosphere.
In cases where the caregiver is alone and solely responsible for a patient, there are several options available that can either augment a lack of caregiver medical knowledge or educate a caregiver on the specific needs of an individual patient. Visiting Nurses are especially skilled at not just caring for a patient at home, but giving an informal but extremely valuable education to family members who are cast as caregivers. While this does not take the place of an extensive medical education, it does impart a fairly good amount of competent guidance to a caregiver. Some Visiting Nurse Associations even provide a 24 hour hotline that caregivers can call with medical concerns which are answered immediately.
So long as the patient is comfortable with the caregiver, and the caregiver is willing to give their best intent to the well being of a patient, the amount of medical knowledge a caregiver requires is minimal and can be learned quickly. More important traits for a caregiver are common sense, compassion, and the ability to give love freely.