The virus that causes AIDS is the trickiest pathogen scientists have ever confronted. It mutates furiously, it has decoys to evade the immune system, it attacks the very cells that are trying to fight it, and it quickly hides itself in your genome.
We know that children who are healthier do not require medical treatment or care, both of which cost time and money. So, by avoiding illness, infants have a greater chance of growing into healthier children who are able to attend school and become more productive members of society.
New vaccines are being developed all the time, which could save many more lives and dramatically improve people's health. And this goes beyond the traditional burden of childhood infectious diseases.
As megacities like Mexico City and Lagos become increasingly common, we could see a rise of the urban epidemic and a new era of infectious disease threatening global health security.
Yellow fever outbreaks are not uncommon. But, as with other infectious diseases, when they occur in urban areas, they can play out very differently - not least in terms of the speed and scale at which they can spread.
With infectious disease, without vaccines, there's no safety in numbers.
In global health, emergency vaccine stockpiles are like the insurance policy you never really wanted to take out: you resent the cost and have mixed feelings about never making a claim. Moreover, given that a stockpile is often a last resort, if you ever fall back on it, you have, in some way, already failed.
Investments in immunization yield a rate of return on a par with educating our children - and higher than nearly any other development intervention.
We've actually eliminated Type II polio in the world, at least as far as we can tell.
As cities get bigger, our best defence will be to prevent outbreaks in the first place by building better public health systems, improving childhood immunisation through better routine immunisation and pre-emptive vaccination campaigns.
I wish we could have state-of-the-art hospitals in every corner of the earth... but realistically, it's going to be a while before that can happen. But we can immunise every kid on earth, and we can prevent these diseases. It's only a matter of political will, a little bit of money and some systems to do it.
No country in the post-colonial era has thrived without first building its capacity to conduct scientific research.
As more and more people adopt an urban lifestyle and cities continue to swell, not only does the risk of urban epidemics increase - something we haven't seen much of for decades - but the need for larger emergency stockpiles can increase, too.
The GAVI Alliance has achieved many things in its first dozen years, but none more important than helping save more than 5.5 million lives and prevent untold illness and suffering.
If you want to know the value of vaccines, just spend some time in a clinic in Africa. The faces of the mothers and fathers say it all: vaccines prevent illness and save lives.
