By Noel Randewich

VILLAHERMOSA, Mexico, Nov 7 (Reuters) - Tens of thousands of Mexicans forced into makeshift shelters by massive flooding are threatened by ailments ranging from colds to cholera, health officials said on Wednesday.

Some 80,000 people from the flooded city of Villahermosa have taken refuge in crammed schools, churches and a multistory parking garage.

Colds, respiratory illnesses and foot fungus have become common, and doctors in the tropical city fear outbreaks of more serious diseases like cholera due to a lack of running water.

"The risk now is infections. There could be an epidemic," said Ramon de Jesus Velarde, the head of Tabasco state preventive health program. He said cholera and dengue fever were the main threats.

Standing water attracts mosquitoes, which can carry infectious diseases like dengue. Cholera is transmitted by contaminated water.

Days of heavy rains last week put most of Tabasco state, including capital Villahermosa, under several feet of water, in what was one of the largest natural disasters in recent Mexican history.

The floods, caused by rivers overflowing, killed at least three people and damaged about $655 million worth of crops, homes, businesses and infrastructure.

Twenty-one people were feared dead in the remote village of Juan de Grijalva, in neighboring Chiapas state, after a huge mudslide more than half a mile (1 km) long fell on it.

Much of downtown Villahermosa was still under water on Wednesday and thousands of people lined up to receive food handouts.

Army and government doctors have given out thousands of injections in the past few days mostly against hepatitis and tetanus. Residents, many wearing only flip-flops on their feet, cut themselves on submerged objects as they waded through muddy waters.

The government was planning a mass grave for thousands of cows, sheep and other livestock lost after farmers' fields were flooded in rural Tabasco.

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