There are fears for hundreds of thousands of Rohingya refugees in Bangladesh after the first confirmed cases of Covid-19 were reported in the camps this week.
The Cox's Bazar camps are home to over 850,000 Rohingya refugees living largely in close proximity to one another and with limited hygiene facilities.
Tearfund said the impact of Covid-19 on the camps "could be devastating". It has been working over the last few months to prepare the camps for the arrival of coronavirus by providing education on how to prevent infection, and installing sanitation facilities.
The Christian development agency is distributing hygiene kits and leaflets in the Rohingya language to raise awareness about hand washing, social distancing and the symptoms of Covid-19.
It has also been cleaning out drains in the camp and distributing food to people already in quarantine.
Sudarshan Reddy Kodooru, Tearfund's Country Director in Bangladesh said: "A case of coronavirus in the Rohingya camps is the news Tearfund has been dreading - but we are not without hope.
"Tearfund and its partners in Cox's Bazar have been racing against time to prepare people in the camps, by teaching good hand washing, sanitation and hygiene. This is their best defence in a place where social distancing is extremely difficult.
"The vital work Tearfund is doing to increase the number of hand pumps and sanitation facilities is more important than ever, as is the work persuading people to use them."
Christian Aid, which is also working with partners in the camps, said that a third of the refugee households do not have soap or regular access to water, and that access to healthcare is also poor, with 29% reporting that one or two members of their family already have an illness requiring medical attention.
The aid agency is warning that the spread of coronavirus around the camps will leave many people unable to earn a living, triggering extreme poverty and hunger.
It is calling on world governments to take immediate action to help the refugees.
Christian Aid's Bangladesh Country Director, Pankaj Kumar said there was "no doubt" coronavirus would spread "rapidly" through the camps.
"Like much of the world we are subject to lockdown rules, but this is disrupting the essential humanitarian aid the refugees rely on, leaving the Rohingya further at risk," Kumar said.
"For a community largely reliant on food, water, healthcare and protection from organisations like Christian Aid and its local partners, this is a grave concern.
"Longer-term effects of the outbreak will be devastating, with little to no opportunities for people to earn a living, resulting in hunger and extreme poverty and increases reliance on humanitarian aid. Women and girls are particularly at risk, with increased domestic violence, child marriage and marriage-related trafficking.
"We are calling on governments and donors, like DfID, to invest in urgent local responses, to ensure that community-based responders, including faith-based organisations, receive direct funds for their work to try and protect the thousands of Rohingya refugees now at serious risk of catching Covid-19."