SINGAPORE - The authorities are investigating after 18 people fell ill from typhoid fever in about three weeks.

The individuals developed symptoms of the bacterial infection between July 13 and Aug 4, the Ministry of Health (MOH) and Singapore Food Agency (SFA) said in a joint statement on Saturday (Aug 17).

All 18 were hospitalised as typhoid fever is typically diagnosed through blood or stool tests done in hospitals.

They are currently in stable condition, the joint statement said, adding that 14 of the patients have been discharged.

In response to queries from The Straits Times, MOH and SFA said that they are currently investigating the cause of the outbreak.

This involves interviewing affected individuals to identify common links and collecting food and water samples for testing.

Household members of those who fell ill have also been tested and advised to seek medical help if they develop symptoms, said the ministry and SFA.

In cases where the affected individuals are also food handlers, they will not be allowed to work until they have fully recovered.

"As an added precaution, MOH has informed doctors through an alert to remain vigilant and report typhoid fever cases."

Dr Leong Hoe Nam, an infectious disease specialist at Mount Elizabeth Novena Hospital, said the uptick may reflect a certain complacency among Singaporeans.

"If the cases are local, it tells you that restaurants are doing a terrible job. If it is overseas imports, it could mean that Singaporeans are ignorant of risks when they travel and they need to be educated," he said.

Typhoid fever is an infection caused by the bacteria Salmonella typhi, and is transmitted through eating food or water contaminated by the faeces and urine of patients or carriers.

Symptoms of typhoid include a prolonged fever which may be accompanied by a headache, body aches, vomiting, diarrhoea or constipation.

Dr Leong said that the disease can be potentially very dangerous for those who are very young or elderly.

It is treatable with antibiotics, and can be prevented by handling and cooking food safely and in a hygienic manner.

Food items that could be contaminated include raw or ready-to-eat foods, like raw (unpasteurised) milk or their products, seafood, and fresh produce including fruits and vegetables.

Correction note: This article has been edited for clarity.