Immunization (or vaccination) is a safe and effective way to provide protection against certain diseases. Immunization schedules are developed to provide the best possible protection from serious diseases. Some vaccines are only given once or twice, some need to be given in a series over a period of time and others require a booster after several years. Public Health Nurses provide provincially funded (free) immunizations to infants, children and all other age groups who meet the eligibility criteria for each vaccine. There is no cost for the vaccines delivered by Public Health Nurses. Some vaccines, including travel vaccines, may be recommended but not provided free under the provincial immunization program. There is a cost for these vaccines and they can be purchased through a private clinic or provider such as a physician, nurse practitioners and certified pharmacist. These providers may also provide some of the publicly funded vaccines to certain age groups. Generally, a referral is not required for immunization. A physician or specialist referral may be required for some individuals whose medical condition or treatment may affect their immune system.