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The answer is, not very. In fact, today's hospitals are the best places to go to get ill – a phenomenon known as "hospital-associated infection". It's a problem with grim statistics: 5000 people die from hospital-associated infections in the UK each year – more than die on the roads; 10,000 die each year in France; and, in 2000, 88,000 died in the USA.

What makes the problem even worse is that those statistics are not linked to the existence of obsolete hospitals from the past two centuries. The facilities we are getting in the current hospital building boom might be no better. In fact, new research suggests that the American-style deep-plan hospitals could be making the problem worse – a situation exacerbated by the lack of knowledge about the spread of infection in hospitals.

"It's vitally important that designers have an understanding of how infection can be transmitted in hospitals so the buildings do not add to the problem," says Dr Clive Beggs, senior lecturer in Aerobiological Engineering at Leeds University. "For example, the recirculation of air in mechanical ventilation systems can increase the spread of airborne pathogens."

This does not bode well for deep-plan hospitals where many rooms are windowless and so have to be mechanically ventilated. Unfortunately, deep-plan hospitals are popular. PFI consortiums like them because the efficient floor-to-wall ratios mean they are cheap to build. NHS trusts are sold on the idea because they believe it is more efficient to have departments adjacent to each other rather than having them strung out like fingers on a long, central street – a model traditionally associated with northern Europe.

Phillip Nedin, a director of the health division of consultant Arup, has worked with Beggs on the spread of infection within hospitals. "In the States, the outbreak of infection is greater than in the UK, even though they have more single rooms. This could be down to mechanical ventilation," Nedin says. He adds that it is much easier to isolate an entire department if it is designed on the northern European model.

Nedin and Begg's computer modelling of airflows in hospitals has revealed how little is known about the airborne spread of disease. Nedin became involved nine months ago because Arup was asked to design an isolation room. "We looked for the guidance but there wasn't anything," he says.

An isolation room is intended to contain highly infectious diseases. It has an airlock, a lobby, a room for the patient and an en-suite bathroom. The room has to be mechanically ventilated to maintain a constant flow of clean air.

Nedin's airflow modelling work revealed some interesting facts. "The relationship between the lobby, the bed and where the doctor stands is very important and doesn't get the attention it should," he says. Air is usually extracted from the lobby to create negative pressure within it, thus preventing airborne pathogens from escaping into the main building. Unfortunately, this pulls all the pathogens in the bedroom towards the lobby entrance. "As you go through the airlock and step into the bedroom, staff and visitors get the full impact of any bugs," Nedin says.

Modelling showed that the ideal solution is a linear arrangement of spaces, with the bedroom sandwiched in between the en-suite bathroom and the airlock. Clean air enters the lobby and passes into the bedroom, then is extracted in the bathroom.

The trouble with this layout is that it uses space inefficiently and is therefore less appealing to PFI consortiums. But Nedin says that the current arrangement puts the lobby next to the en-suite bathroom, allowing the clean air to flow straight into the bathroom without mixing properly with the infected air around the patient.

Nedin says there are other barriers to more effective hospital design. For a start, very few people are qualified to tackle the relationship between building design and the spread of infection. "One of the most difficult areas for us is the relationship between the engineer and the microbiologist, as they speak a different language," says Nedin. "We are seeing a new discipline emerging – the engineering microbiologist who will understand bugs and engineering systems."

Another issue is getting all the stakeholders involved in a hospital project, including clinical staff, the NHS trust, contractors and construction consultants, to sit around a table and agree common objectives. "The stakeholder group is huge. There can be 12 people around a table and they don't all understand design," Nedin says. "They all want different things and don't understand the trade-offs involved."

He cites an example in which lack of technical understanding caused problems. Arup prepared a walk-through visualisation of a deep-plan hospital that had very low ceilings because so much space was taken up by ventilation ducting. The stakeholders said they really disliked the low ceilings, even though they had already seen the drawings and accepted them as reasonable.

Nedin reckons the ultimate answer is a more holistic – and tougher – approach to preventing the spread of infection. A key part of this is ensuring that ancillary staff do their jobs to a high standard.

"One of the things we like to push is zero tolerance," he says. "How can you expect people to wash their hands if there is rubbish piled up and dirty laundry everywhere?"

Arup is working with the Northern Ireland Health and Social Services Estates Agency to tackle this problem. A system that sucks dirty laundry or rubbish from rooms in the hospital through twin 400 mm diameter pipes is being installed at Altnagelvin Hospital in Londonderry – the second such system in Europe.

Another aspect of keeping hospitals clean and well-maintained involves creating a pleasant working environment, something Nedin believes is incompatible with windowless deep-plan hospitals. He says there is some evidence that American architects are moving away from deep-plan on their home patch.

Encouragingly, NHS Estates recognises that this is a problem that demands urgent attention. "It's a key issue for us, says Darryn Kerr, head of engineering at NHS Estates. At the end of last year, the chief medical officer issued a paper called Winning Ways – Working to Reduce Healthcare-Associated Infection in England. "We have a number of initiatives supporting Winning Ways including a complete review of our suite of guidance to make it more manageable and up to date," Kerr says. "In the long term, the idea is to have best practice guidance."

Kerr, Nedin and Beggs agree that most of the research that has been carried out into infection spread in hospitals so far has been inconclusive. "We need to keep an open mind on the ventilation issue, but we must keep in mind that any changes must be made on sound scientific evidence or we could make the situation worse," Kerr warns. He hopes to have new design guidance in place within 18 months. Unfortunately, by the time any conclusive research and guidance is published it may be too late for many hospitals designs.