Cameron Devenney was first taken ill when he was 11 months old in July 2007, but it took a further four months to find what was wrong with him.

It was first thought he had leukaemia but his symptoms, including an enlarged spleen and abnormal bone marrow, confused doctors at the Good Hope Hospital, near Birmingham.

Further tests were carried out and his parents, John and Anne Devenney, found they were carriers of the hereditary condition hemophagocytic lymphohistiocytosis (HLH).

Mrs Devenney, 35, who is originally from Wakefield, said: "Because it is so rare, people didn't understand how poorly he was. There are problems diagnosing it because it presents symptoms similar to other diseases."

Cameron needed chemotherapy and a bone marrow transplant, and he was transferred to Birmingham Children's Hospital.

In the search for a donor, Cameron's sisters, Caitlin, nine, and Hollie, 12, and his brother, Ciaran, six, were tested and were all a 100 per cent match. Ciaran was chosen as the donor.

Mrs Devenney, who lives in Tamworth, Staffordshire, said: "He was so brave and he really wanted to help his brother."

The family believed the transplant would save Cameron. But his health deteriorated and he died on April 22 this year, aged 20 months.

His death has led his parents and his aunt, Carole Furlong, of Storrs Hill, Ossett, to begin fundraising for charity the Histiocytosis Research (HR) Trust, which funds research into rare blood disorders.

Miss Furlong, 48, said: "We just want to raise awareness of the condition because more research needs doing into it."

People can donate to the campaign by logging onto www.justgiving.com/cameronssmile