Yet only a year ago, says Karen, Sam was "ridiculously healthy". She adds: "He never had anything wrong with him, apart from some mild asthma, was doing well at school and loved football." Then, late last year, he fell downstairs. He continued to have falls, and Karen eventually took him to Great Ormond Street Children's Hospital.
"Has he been exposed to measles?" asked the doctor. "No, he has never had measles," Karen replied. "No," the doctor said, "I meant, has he had a recent measles injection?"
Sam had indeed been vaccinated against measles the previous November, in response to a Government campaign. The Department of Health urgently warned parents last year that unless there was a mass re-vaccination programme, the country risked a measles epidemic in 1995, infecting between 100,000 and 200,000 children. Around 50 children, mostly of secondary school age, would die, it said.
The campaign duly went ahead, with 8 million children aged up to 15 being given combined jabs against measles and rubella (German measles). Then children started to fall seriously ill. Sam was one of them.
One parents' group, Jabs, says that so far 85 families have reported that a child has fallen ill after the November inoculation, and of these 40 seemed to have long-term illnesses, including epilepsy, rheumatoid arthritis, Guillain-Barre syndrome, encephalitis, and post-viral fatigue syndrome.
The Department of Health is resolutely denying that the vaccination campaign had any long-lasting effects on any children. But now doctors are raising disturbing questions about it, and about why Sam and other children have fallen so ill. Richard Nicholson, editor of the Bulletin of Medical Ethics, argues, after studying Government reports, that there was no proof that a measles epidemic was about to start. Further, the risk from rubella was minimal to all but pregnant women - and 99 per cent of teenage girls over 14 were at that time immune from rubella. The estimate of around 50 deaths, according to Dr Nicholson, was based on "improper use of statistics and out-of-date death rates".
In short, Dr Nicholson believes that the Government was scaremongering. Virginia Bottomley, then Secretary of State for Health, "hoped to make political capital from the campaign. To recognise a threat to the nation's children and to take decisive action ... could only improve her standing."
Karen, Sam's mother, was one of the parents who raised questions, in writing, about the immunisation. Would it be safe for him, given his asthma? she asked. She also mentioned that another of her children was epileptic. There was no reply from Sam's school, but her GP, she says, told her not to worry.
Now, her life has become a nightmare. She is exhausted from constantly caring for Sam and scarcely has time to see to her other three children. She blames herself for letting her son be immunised; her consultant pediatric neurologist at Great Ormond Street Hospital for Sick Children told her, she says, that in her view the vaccine had triggered off her child's illness - sub-acute sclerosing panencephalitis, (SSPE), a neuro-degenerative disease.
Although the Department of Health insists no children are now suffering after-effects of the vaccination, Mrs Bottomley admitted, in reply to a letter from another mother, Susan Hamlyn, whose son, Francis, had gone down with juvenile arthritis a month after being immunised, that "there have been a small number of late-onset and longer-lasting suspected adverse reactions that have occurred post-MR (measles and rubella) vaccination. It is of course of the utmost importance that these be investigated."
She mentioned that "abnormality of the joints ... is reported after rubella vaccination". She said that the Medicines Control Agency was "following up many of the ... reports of suspected adverse reactions".
Jackie Fletcher, the secretary of Jabs, said: "Some children's lives have been severely damaged as a result of last year's campaign. We will not know all the side-effects for many years. It is morally wrong for the Department of Health to keep on saying there are no consequences without researching these consequences.
"Ideally they need to repeat the 1988-1993 examination of hospital admission and GP records, so that there can be an accurate assessment of the numbers of children affected."