Cryptosporidiosis. The first graph shows that notifications are seasonal, with variable peaks in the warmer months. When the causal parasite, Cryptosporidium, is ingested (usually following direct contact with another person or an animal with the infection, or by swallowing contaminated water or food) it can cause diarrhoea, vomiting and abdominal cramps the last several days or weeks. Outbreaks are often associated with swimming in pools. Cryptosporidium is highly resistant to chlorine and very small. If a swimmer introduces it into it a swimming pool, it can slip through filtration systems and survive for many days. If a swimmer ingests even a small amount of contaminated pool water, they can become ill. Variations in the number of people infected in each outbreak may relate to both the number of people exposed to the infection, and their level of immunity (which probably lasts only a few years after infection).
The second graph shows that notifications are largely among small children and to a lesser degree people of child rearing age. Small children may be more likely to ingest water when in a pool, and develop illness, leading to person to person transmission within their families.