There are a number of diseases that, in most cases, can be prevented through protection from timely immunisation. In New Zealand, the National Immunisation Schedule protects children against eleven such diseases.
Each disease carries its own set of risks. Some are more serious at certain ages, whooping cough for example is most dangerous in very young babies. Some are rare but very serious for most people who get them, such as tetanus.
Some diseases, like measles, are very contagious although not everyone who gets it will be seriously ill. It is impossible to predict who will get seriously ill and who won’t.