Ebola. Zika. Both diseases that were unknown to many until recently. But there have been huge outbreaks of both – and each time scientists and global health experts were caught off guard.

In this week’s Scrubbing Up, Dr Seth Berkley, CEO of the Gavi, the Vaccine Alliance suggests Ebola and Zika may be followed by other public health emergencies fuelled by other lesser-known diseases.

First it was Ebola and now Zika; two official World Health Organization (WHO) Public Health Emergencies of International Concern within as many years.

Both diseases have been known about for decades, and yet in both cases no vaccines or drugs were available when we most needed them. So what’s going on?

Is this just a terrible coincidence, being caught off-guard like this twice in such quick succession, or is it part of a worrying trend and a taste of things to come?

At first glance it wouldn’t appear that the two diseases have much in common. One is difficult to catch but a ferocious killer, while the other spreads with ease but is relatively harmless to the vast majority of people infected.

Yet, in both cases there is something novel, either in the way the virus has spread or in how it affected people which has made the outbreaks more of a threat.

In global health security terms that is a real concern, because such sudden changes of modus operandi can not only make public health threats even more difficult to predict or anticipate than normal, but also make all the difference between a localised outbreak and global pandemic.

Even more worrying is the fact that with changing trends in human and animal migration, increasing urbanisation, the density of mega cities, the rise in antimicrobial resistance and climate change, such threats could become increasingly more common.

In the case of Ebola, what changed was its ability to spread.

Historically Ebola’s aggression has been its own worst enemy; the virus often immobilising and killing its hosts before they had the opportunity to infect others, limiting its spread mainly to contact with the deceased.

Because of this, for decades it remained a relatively low impact disease, confined to small outbreaks in remote and relatively unpopulated rural regions.

What changed in West Africa was that for the first time it was able to reach more densely populated urban areas, increasing its ability to spread almost exponentially.

With Zika it was different. It had been believed to be a relatively benign disease, producing only mild flu-like symptoms – if any at all.

Because of this there was little concern about the spread of this mosquito-borne disease as it crossed continents.

But now with a Zika outbreak suspected as the most likely cause of a sudden spike in cases of microcephaly in Brazil – which can cause babies to be born with abnormally small heads – we have another global health emergency on our hands, particularly if reports of sexual transmission prove valid and its spread is not limited to mosquitoes.

If Zika is a factor with microcephaly, it is not entirely clear why. In the seven decades since Zika was first discovered, such horrific complications have never before been observed.

A form of nerve damage, called Guillain-Barré syndrome, has been seen a small number of people, and in the case of pregnancies there were 17 cases of malformations of the central nervous system in foetuses following an outbreak in French Polynesia in 2014.

However, even then Zika was not implicated until recently, and only after the alarm was sounded in Brazil.

So, why now? It could simply be something we only see as a result of scale – 1.5m cases of Zika in Brazil, compared to just 30,000 in the worst previous outbreak.

Or it could just be that surveillance in Brazil was good enough to detect it, picking up both increases of Zika and microcephaly immediately, compared to West Africa where poor health systems meant it took three months before Ebola was first confirmed.

Or it could be that the virus has mutated to a more virulent strain.

Regardless, the fact remains that it could take years before we establish a conclusive link with microcephaly, and possibly even longer before we understand the epidemiological factors leading to its sudden emergence now.

What is clear is that in addition to mosquito control in affected countries what may also be needed is another new vaccine.

But unlike Ebola, we don’t have several candidate vaccines lined up, waiting in the wings which we can rapidly steer through clinical trials.

Thanks to Ebola, industry has now been faster to react and commit to develop a vaccine or adapt existing ones, but it will still likely be years before one is ready.

However, why does it take a global health emergency for us to even realise no vaccine exists in the first place?

Part of the problem is that for some serious diseases there is simply no profit in prevention, which means that if we want to avoid epidemics we cannot expect industry to provide the solution.

Instead governments, public funders and private donors need to share the costs, and they need to do so now, rather than waiting until the next epidemic.

The good news is that we now already have an idea of where to focus our attention. In December the WHO brought together scientists and clinicians who came up with an “initial list” which reads like a most-wanted of the worst eight diseases, including Ebola and other haemorrhagic fevers like Marburg and Lassa fever.

They also flagged a sub-set of three other serious diseases that also needed urgent attention, which included Zika.

And therein lies the point. None of the diseases on the list were particularly surprising.

They are known threats, it’s just that they are not big enough threats to have warranted the world to rally round and put in place the incentives to develop vaccines, at least not yet.

What Zika and Ebola have both taught us is that we can’t assume pathogens will continue to behave the same way.

We need to stop waiting until we see evidence of a disease becoming a global threat before we treat it like one.