Tens of thousands of Mexicans affected by widespread flooding in the state of Tabasco are at risk of epidemics of diseases, officials have said.
Thousands of people have been receiving food handouts
The health ministry said teams of medical staff were to travel the area to administer injections against hepatitis, influenza and tetanus.
There are also fears of an outbreak of other diseases, such as cholera and dengue fever.
More than a million people have been affected by last week's flooding.
At its worst, an estimated 80% of the state was under water.
Much of the centre of the state capital, Villahermosa, is still covered in water, reports say.
Tens of thousands of residents are living in makeshift shelters in schools and churches, and in cramped conditions, colds, respiratory illnesses and skin infections have become common.
At least four people were killed in a landslide in Chiapas
"The risk now is infections. There could be an epidemic," said Ramon de Jesus Velarde, the head of Tabasco state's preventative health programme.
Mexican interior minister Francisco Ramirez Acuna said fumigation of waterlogged areas would start in the next few days.
Earlier this week, Mexican President Felipe Calderon announced an emergency fund of more than $600m for the flood-ravaged state.
In neighbouring Chiapas state, four bodies have been recovered after a mudslide hit the remote village of San Juan Grijalva.
Rescue workers said they held out little hope of finding a further 21 missing people.
"We have to call them missing, but it is hard to hold out much hope of finding them alive," Chiapas Governor Juan Sabines told a local radio station.