Pupils from South Woodham's William de Ferrers School were being reassured this week after it was revealed a teenager is suspected of suffering from meningococcal meningitis.
A Chelmsford baby also currently has the killer brain disease.
The North Essex Health Authority say that the immediate families and close contacts have been offered antibiotics.
A spokeswoman declined to reveal the sex or ages of both victims but said both cases were "isolated" outbreaks and unconnected.
The teenager is being cared for at Broomfield Hospital and the baby at St John's Hospital, Chelmsford.
Mr Dennis Parry, headmaster of William de Ferrers School, South Woodham Ferrers, confirmed he had written to all Year 11 parents telling them there was no need to keep their children away.
He said: "I have passed on a letter produced by the Health Authority designed to allay concerns, but giving details of symptoms and a number to ring if they need additional advice or reassurance."
Symptoms of meningitis include fever/vomiting, drowsiness or losing consciousness, severe headache, dislike of bright lights, stiff neck (not always present in young children) and a rash (not present in all cases).
Symptoms can appear in any order and septicemia can occur with or without meningitis.
For more information contact the Meningitis Research Foundation 24 hour helpline on 0808 800 3344.
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