In November of this year, alarm bells sounded when 23 people fell sick and one died as a result of severe diarrhoea in a health clinic in Tangsee, a town with a population of around 20,000. The severity of the illness and the town's location - at the mouth of a river that supplies untreated drinking water to the entire subdistrict - raised fears of an epidemic.

It is 8am and the rain is bucketing down with such fury that you can barely hear the nurse as she outlines the team's tasks for the day. All across town, families huddle together under their temporary shelter and look on with resignation as water creeps to their front doors.

It is the rainy season in Aceh, a time when even the most robust roofs leak and entire districts are transformed into floating villages. It is also the time when the risk of epidemics is greatest, particularly for those who have been displaced by the tsunami.

The tsunami made almost half a million people in Aceh homeless. Aid organisations and the Indonesian government rushed to the rescue, rapidly erecting barracks into which the majority of people were moved. It was an impressive effort, but one that focused on speed rather than sustainability. Now, almost a year on, hundreds of thousands of people still live in tents and temporary shelters that are coming under pressure from the relentless onslaught of rain.

To reduce the risk of any outbreaks of disease, MSF has launched hygiene and health promotion activities in some of the most at risk locations in two districts of Aceh. MSF nurse Nadje de Groote leads health and hygiene activities in Pidie District, where 22 000 people still live in 44 relocation sites known as barracks. MSF has been running mobile clinics and mental health programmes in the barracks since March.

"The idea was to monitor the health situation of people living in temporary shelters and do hygiene promotion in as many barracks as the teams could handle," says Nadja. "We soon found in several of the camps a risk of water and vector bourne diseases due to the high levels of rainfall and the degradation of water and sanitation conditions."

The pools of stagnant water collecting around many of the camps are a perfect breeding ground for mosquitoes carrying malaria and dengue, both of which are endemic to Aceh. The flooding of latrines and septic tanks also pose other risks including diarrhoea (one of the most common causes of death in children aged under five), typhoid, scabies, worms and eye infections. Hygiene promotion teams seek to prevent outbreaks of disease by encouraging good health practice through the PHAST method.

The PHAST method focuses on fostering community participation and responsibility in health and hygiene through training, talking groups and interactive information sessions. All members of the community are encouraged to join in on activities, and sessions are led by Acehnese health staff whose knowledge of local practice and language skills help identify risks and find suitable methods of addressing them.

"It's amazing how enthusiastic the people are and how many attend these sessions," says Nadja. "People are genuinely and understandably concerned by disease and want to do what they can to protect their health."

Such activities have been proven to help reduce outbreaks of disease. Nevertheless, any success depends on minimum standards of clean water and sanitation. Although there have been no epidemics in any of the barracks where MSF works and people living there are considered to be in quite good health, assessments found that in three camps, water and sanitation conditions posed easily avoidable health risks. MSF quickly stepped in to intervene.

"The sanitation situation in some of the barracks was unacceptable to MSF," explains Nadja. "In one, Arusan barrack, Kembang Tanjong, you have almost 1,700 people living in cramped conditions, but the latrines had become unusable and waste water had collected in pools around the site, where children play and animals drink. It was a real source of concern for me. There was no crisis, but these were health risks that should be avoided."

MSF quickly assembled a logistics team, and prepared a water and sanitation expert to oversee any activities. They also launched an advocacy strategy to bring in other specialist aid organisations. Fortunately, after successful advocacy by MSF, another NGO with the necessary heavy equipment and expertise already on site decided to join forces with MSF, and construction in the camps is presently underway.

Elsewhere across Aceh Province MSF has continued to watch closely for any outbreaks of disease. Mortality and morbidity data is collected from health clinics in all areas where MSF has a presence and the health system is supported through supervision, training and the provision of medical supplies. Particular attention is paid to proper case diagnosis and treatment and should any worrying health indicators appear, MSF is ready to respond.

This is what happened in November of this year. Alarm bells sounded when 23 people fell sick and one died as a result of severe diarrhoea in a health clinic in Tangsee, a town with a population of around 20,000. The severity of the illness and the town's location - at the mouth of a river that supplies untreated drinking water to the entire subdistrict - raised fears of an epidemic.

MSF immediately initiated an emergency intervention. A medical team disinfected the health clinic and set up an isolation ward, health and hygiene promoters visited surrounding villages, educating communities about risks of disease, and, in case it was cholera, a kit to treat 625 people was put on standby. Fortunately the epidemic stabilised and there were no further cases. However MSF has maintained close contact with health officials and community leaders in this area and remains ready to respond again if another outbreak occurs.

A month earlier MSF also responded to a measles outbreak in the mountainous sub-districts of Mane and Geumpang. Since December, almost all aid in Aceh has been focused on the tsunami hit coastal regions, but MSF assessments have found that years of conflict have led to a deterioration of the health system of inland regions, particularly in terms of vaccination coverage. MSF decided to run a vaccination campaign in order to prevent further spread of the disease, vaccinating 2809 children between the ages of six months and fifteen years.

In addition to outbreak response, MSF programmes focus on medical needs that are not yet being covered by the health system. In Lamno, a village in Aceh's west coast, MSF is treating patients with tuberculosis and has set up surgical facilities in the regional health clinic. In Meulaboh, Sigli and Takengon MSF is bringing health care to areas previously made inaccessible by conflict. Across Aceh MSF is running mental health programmes, providing psychological support to those devastated first by war, then by conflict.

MSF's activities are a reflection of the enduring medical needs.

"Over the course of the year the health system in Aceh has made a good recovery and in most areas is now able to meet the most pressing needs of the population. Consequently the number of MSF programmes is expected to decrease," says MSF's medical co-ordinator Dr Patrick Dechamps.

"However, for those still living in temporary shelter or who have been isolated through conflict or poverty, health risks remain. MSF will continue to monitor the situation of these people through its health surveillance activities and be prepared to respond to any emergencies that might occur."