Image copyright Getty Images Image caption The WHO estimates one in 25 people has at least one STI

One million new sexually transmitted infections (STIs) occur every single day, the World Health Organization has estimated.

That means more than 376 million new cases annually of four infections - chlamydia, gonorrhoea, trichomoniasis, and syphilis.

The WHO highlights a lack of progress in stopping the spread of STIs, and says its figures are a "wake-up call".

Experts are particularly concerned about the rise in drug-resistant STIs.

The WHO regularly evaluates the global impact of the four common sexually transmitted infections.

It looks at published research and collects reports from its workers in countries around the world.

Compared with its last analysis in 2012, the WHO reports "no substantive decline" in the rates of new or existing infections.

It suggests around one in 25 people globally has at least one of these four STIs, with some experiencing multiple infections at the same time.

The figures suggest that among people aged 15-49 in 2016 there were:

156 million new cases of trichomoniasis

127 million new cases of chlamydia

87 million new cases of gonorrhoea

6.3 million new cases of syphilis

Trichomoniasis is caused by infection by a parasite during sex. Chlamydia, syphilis and gonorrhoea are bacterial infections.

'Harbingers of a wider crisis'

STI symptoms can include discharge, pain urinating and bleeding between periods. However, many cases have no symptoms.

Serious complications can include pelvic inflammatory disease and infertility in women from chlamydia and gonorrhoea, and cardiovascular and neurological disease from syphilis.

If a woman contracts an STI when she's pregnant, it can lead to stillbirth, premature birth, low birth-weight and health problems for the baby including pneumonia, blindness and congenital deformities.

Dr Peter Salama, of the WHO, said: "We're seeing a concerning lack of progress in stopping the spread of sexually transmitted infections worldwide.

"This is a wake-up call for a concerted effort to ensure everyone, everywhere can access the services they need to prevent and treat these debilitating diseases."

Practising safe sex, particularly through condom use, and better access to testing are both crucial, the WHO says.

In terms of treatment, bacterial STIs can be treated and cured with widely available medications.

But syphilis treatment has been made more difficult because of a shortage in the specific kind of penicillin needed, and there has been an increase in cases of so-called "super-gonorrhoea" which is almost impossible to treat.

Dr Tim Jinks, head of Wellcome's Drug Resistant Infection programme, said: "Untreatable cases of gonorrhoea are harbingers of a wider crisis, where common infections are harder and harder to treat.

"We urgently need to reduce the spread of these infections and invest in new antibiotics and treatments to replace those that no longer work."