The risk of developing deep vein thrombosis or blood clots in the lung doubles after infections, a study says.
The Lancet study of 3 million GPs' records found 7,278 people had suffered DVT and 3,755 a clot on the lung.
The London team found those who had suffered either respiratory or urinary infections were nearly twice as likely to develop blood clots two weeks later.
They suggested the clots could have been caused by inflammation in the blood vessel linked with infection.
DVT contributes to around 10% of deaths among hospital patients each year.
They occur in the leg veins but can also break off and lodge in the small vessels in the lung.
If this occurs the condition is called a pulmonary embolism, which is fatal in about a third of cases without treatment.
The team led by Dr Liam Smeeth, from the London School of Hygiene and Tropical Medicine, said there was already evidence to suggest infection increases the risk of heart problems.
They wanted to look at whether infection could trigger DVT or pulmonary embolisms.
We suspect that infections cause an inflammatory effect on the lining of the blood vessel wall
So they looked at what happened to people who had experienced either acute infections in the urinary tract or in the respiratory system - such as influenza, bronchitis and pneumonia.
They found that while the infection was on-going there was no impact on the risk of developing these blood clots.
But several weeks after the infection, the risk appeared to double.
Dr Smeeth said: "We suspect that infections cause an inflammatory effect on the lining of the blood vessel wall - or endothelium.
"But the blood clots could equally be caused because as people are ill and go to bed they stay still for longer."
DVT has commonly been associated with people sitting still in cramped conditions on aeroplanes.
Dr Smeeth also suggested that dehydration associated with the infection could be thickening the blood a little bit.
However, he added: "The evidence clearly established that infection is a risk factor."
This should be taken into consideration along with other risk factors such as cancer and pregnancy when considering who is at risk of DVT and pulmonary embolism, he added.
Dr David Keeling, Royal College of Physicians advisor and expert in DVT at the Oxford Haemophilia Centre and Thrombosis Unit, said the research was interesting.
He said: "Infection appears to double the risk of DVT and pulmonary embolism for a couple of months.
"It is interesting that there is a smaller increase in risk for up to a year.
"A reduction in mobility may play a role but I suspect that activation of the coagulation system or excessive clotting of the blood by inflammation is more important."