By the time you see them they have conditions that would have brought any other person to casualty two or three days ago

Roger Williams, of Crisis

They also suffer from a variety of respiratory infections, including TB, sexually transmitted diseases like HIV and hepatitis B and C, as well as a variety of skin problems such as impetigo, scabies and warts.

Foot problems are one of the greatest sources of infection among the homeless, who are almost constantly on their feet and often wear inappropriate footwear and rarely wash their feet or cut their toe nails.

These conditions range from skin disorders and wounds to trench foot and ulcers which can become infected and lead to gangrene, amputation and even death.

Trench fever spread rapidly in the cramped trenches of WW1

But experts say that despite their myriad problems the homeless are not always getting the care they need.

They want to see better access to primary care services and for hospitals to treat the homeless as in-patients to ensure recovery.

Roger Williams, a registered nurse and health visitor and medical co-ordinator for Crisis, said homeless people tended to wait until their problems were chronic before seeking help.

True story

One man walked all the way to the Crisis Open Christmas shelter and when the medical team looked at his feet his toes stayed in his sock when they took it off. He needed life-saving surgery for gangrene of the foot

"Because they have a lack of healthcare they tend to hang on and hang and don't go to see a doctor until they are profoundly ill.

"And by the time you see them they have conditions that would have brought any other person to casualty two or three days ago."

He said diseases spread rapidly through hostels and that an increase in asylum seekers was introducing a number of new tropical diseases into the homeless community.

Skin problems and ulcers are among the most common problems

A spokeswoman for homeless charity Shelter said she was not surprised by the high levels of illness found.

She said many homeless people did not have access to medical care and so their illnesses often go undetected and can rapidly deteriorate.

Improving the health of the homeless

Routine vaccinations against tetanus, diphtheria, hepatitis B and A and flu Free condoms and syringes to stop the spread of sexually transmitted and blood-borne diseases Special care given to footwear and weekly foot cleaning A complete change of clothes weekly

"Many homeless people do not have access to primary care. But there is a lot that could be done in terms of preventing people from becoming ill.

"The illness in many cases is due to poor health conditions and nutrition."

The study is published in The Lancet: Infectious Diseases.