Ten months after Haiti was rocked by a massive earthquake that killed 240,000 people and left 300,000 injured, this desperately weakened state again faces a deadly threat. Cholera has struck.

In the past two weeks more than 4,700 people have been reported as being infected with cholera. A bacterial infection of the intestines, cholera causes chronic diarrhoea that leads to critical dehydration. It can kill in hours.

It is difficult to contain an epidemic, as the diarrhoea contaminates water sources, and without careful hygiene precautions the victims can contaminate those they come into contact with. Cholera kills the weakest first, but it can move fast and affect all.

I am writing from Port-au-Prince where I have been based since 13 January – the day after the earthquake struck.

Goal responded immediately to that disaster and is again responding quickly as part of a massive humanitarian effort to hold back the spread of this epidemic.

On 21 October a small number of cholera cases were reported. As of Thursday, 4,722 people have been hospitalised and there have been 303 cholera-related deaths.

Port-au-Prince is squeezed between the sea and the slopes of the steep hills that hang over the city. The population swelled rapidly to 4.5 million in a few short years. Shoddily built box houses pepper the city without any consideration to essential infrastructure such as roads or drainage.

When the earthquake hit, some 300,000 people were made homeless and today they are living in about 2,000 makeshift camps in cramped conditions with little resources. If cholera takes hold here it could move through these settlements at an exponential rate and has the potential to kill thousands in days.

Goal, Concern, Haven and other Irish aid organisations are working constantly to prepare the population of this city to defend against the disease.

It is not difficult to avoid contracting cholera and it is not difficult to treat. But if simple steps are not taken by all of the communities, then cholera will move rapidly, contaminating water and food and through contact between the infected and the vulnerable. It will kill and it will kill quickly.

Tomorrow is All Saints Day in Ireland. In Haiti it is called 'the day of the dead'. Goal is working hard on the ground in Haiti to make this year's festival a celebration of the day of the living.

Darren Hanniffy is GOAL's Country Director in Haiti