fanjo - one of the complications of flu is to develop a concurrent bacterial infection. This happens with seasonal flu too. Usually flu is not the soul cause of death but a contributory factor. Tonsillitis is not an underlying health condition.
It sounds as if the fact she was diagnosed with SF may have caused less pro action into looking for another cause for the fever esp as the advice at that time was stay home and get tamiflu~this may have delayed treatment.
Patients who die from flu usually do die from a bacterial infection to which they become susceptible when their immune system is knackered by said flu virus. Overwhelming infection is not a rare event. When at secondary school and university I knew two people who died from that - one was following tonsillitis. Infection can move fast - which is why the advice on here is so often to get off the internet and down to A&E when people post about poorly children. The children's matron in the hospital where I worked told me about one case where parents were bringing their child in to be seen and the child died in their arms whilst they were carrying them up to the ward from the car park. They hadn't seemed all that ill at the Gp practice just a little while before I'm not saying that to frighten anybody - i've got a feverish two year old here myself tonight, just to say that modern medicine lulls in to a false sense of security as far as our health goes. We're all bust stressing about swine flu because it's new - but the bugs that can kill are always with us. we're better at treating them now than 50 or 100 years ago - but they've never gone away.