It takes two days to realise there's something wrong. Then the vomiting begins, followed quickly by crippling diarrhoea. Two days of incubation in a child's body is enough for the virus to do its work. If they are unable to fight it they will end up so severely dehydrated they will have to go to hospital, they could end up on a drip, of several weeks, at which point the child's brain and development will have been so severely affected, and in some cases unfortunate enough to lose their life.
This is the case for children who get infected and whose immune system cannot fight rotavirus. Rotavirus kills more than a million children every year. This and other diarrhoea diseases are the second biggest killer globally, costing more children's lives than HIV, famine or war. And in most cases it can be prevented.
Immunisation is one of the world's biggest success stories - eradicating smallpox and reducing polio to be endemic in only four countries.
And the story gets better. Vaccines that prevent rotavirus and pneumonia are now being rolled out in the world's poorest countries. So far we're on track to immunize over 250million children and save four million lives thanks to new commitments by donors at the Global Alliance of Vaccines and Immunizations (GAVI) when they met in London a year ago to this day.
This success is not only due to new science - it's a remarkable story about collective political will - and what can happen when world leaders and citizens speak with one voice.
Last year, when the funding pot was dangerously low, members of the public, charities, ministries of health, MPs, and ultimately donors rallied together to do everything possible to cover the cost of providing life-saving vaccines.
It was a cost that the world could not afford to miss, and the UK public agreed it was worth fighting for. Save the Children created a 'Mr Vaccine', a popular spoof super hero Twitter vaccine 'that'd been growing in a factory for two years - and liked travel and saving lives'.
Bloggers followed vaccines to Mozambique where they met health workers and delighted parents. Across the UK campaigners lobbied their MPs, more than 46,000 people signed a petition asking the UK government to fund vaccines and 100 MPs signed a motion to support our campaign.
Thankfully, over £2.8billion was pledged at the GAVI conference last year, the target was overshot by £283million. And in the last year not a single donor reneged on their promises.
As a result more than 60million children are being vaccinated - preventing what we estimate to be 1million future child deaths.
We are continuing to campaign on immunization. Vaccines don't inject themselves and we need to ensure there are health workers in place, that the costs of the vaccines are reduced and that there are accountable systems in place to reach the most marginalised children. But today it's a reminder that we can do something. It's a heart-warming testimony that when enough people speak out, the right thing is done.
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