There are other conditions which do mimic it, including severe dandruff - just to mention one. It really can't be completely avoided, though it is also not extremely common. Fungus is present in almost all areas of our environment. Soil, animals, carpeting, shoes, people, and a whole host of fungal sources are present in our everyday lives. It has been said, however, that transmission from adult animals is quite rare.
Older people are much less likely to contract ringworm than younger people.
For after having had the problem before, they may have developed some level of immunity which is protective. However, immunity to a fungus occurs far less efficiently than to bacteria and viruses, so recurrent attacks do occur. To complicate things a bit further - because there are a number of different fungus which cause these infections, an immunity to one may not convey an immunity to another. Whenever there is an area of hair loss on an animal, ringworm can be suspected. Even a negative exam when a child clearly has the condition - can be a reason to take the pet to the veterinarian for evaluation.