Statistics revealed following a Freedom of Information Act request by the Conservatives have found nearly 20,000 pest infestations in the NHS between January 2006 and March 2008.
The worst problems were at Nottingham University Hospitals NHS Trust, which had more than 1,000 pest incidents over the period.
In total there were 19,922 incidents or suspected incidents of pest infestations over the period.
Most common pests were ants (101 trusts), mice (98 trusts), wasps (91 trusts), rats (83 trusts) flies (77 trusts) and cockroaches (75 trusts).
There were eight reported cases of maggots - in one case nesting in a patient's slippers.
The Conservatives cited studies where small insects have been shown to spread drug-resistant pathogens.
But health minister Ivan Lewis hit back. He said: ‘The claim that insects spread hospital acquired infections is entirely unproven.
'There is no evidence of their carriage of antibiotic resistant bacteria being a hazard to patients. Despite this we expect hospitals to take continued action to tackle pest problems.’