It starts off as a small, painless lump on the skin.

Key points: The rate of Buruli ulcer infection in Victoria has almost quadrupled in four years

The rate of Buruli ulcer infection in Victoria has almost quadrupled in four years The state had more than 330 cases in 2018

The state had more than 330 cases in 2018 Medical researchers are perplexed as it is typically a tropical infection

Over several weeks, it swells into a much larger and more obvious red bulge.

Eventually, the skin breaks open, exposing a raw and pus-filled wound that continues to grow.

This is how a Buruli ulcer forms, the result of flesh-eating bacteria that is infecting hundreds of Victorians.

WARNING: THIS STORY CONTAINS GRAPHIC IMAGES

"It was really scary," university student Jacinta Mazzarella said. "I was really worried I might lose my limb."

Several months ago, the 18-year-old developed a Buruli ulcer that was bigger than a 50-cent coin on her left ankle.

She believes she may have contracted the disease last summer at her family's holiday home on the Bellarine Peninsula, south of Melbourne.

But no-one can be certain how she came to suffer from the ulcer because scientists and doctors simply do not know how the disease spreads.

Fortunately, medical advancements have meant treatments are much more effective than in the past, and Ms Mazzarella's skin is starting to heal after extensive medical care.

"I couldn't work because I wasn't allowed to stand on it for a long period of time, so I had a good four, five months off," she said.

"I'm a dancer — I had to stop dancing."

CSIRO researchers like Kim Blasdell are trying to learn more about the cause. ( ABC News: Joanna Crothers )

Ulcers getting bigger and more severe

Over the past four years, the annual number of infections reported in Victoria has almost quadrupled.

Diagnoses in Victoria: 2014 2015 2016 2017 2018 89 107 182 277 336 Source: Department of Health and Human Services

Cases are also getting more severe, leading to larger wounds that can take longer than a year to heal.

"It's got a toxin that actually does three things," explained Associate Professor Daniel O'Brien from Barwon Health.

"One is dissolve flesh.

"A second thing is it produces is an anaesthetic agent so a lot of the time, especially early on, patients don't feel it.

"The third thing is it actually paralyses the immune system."

The flesh-eating bacteria, Mycobacterium ulcerans, is related to the infection that causes leprosy.

It stays dormant in an infected person for months before a wound starts to bulge.

"It is an aggressive disease that's not easy to treat," Dr O'Brien said.

"It's not your usual bacteria, so normal antibiotics don't work.

"I'm seeing a higher proportion of severe disease — that is, much worse cases than we did about five years ago.

"The ulcers are more aggressive, they're bigger, it causes more tissue destruction, but it also means treatment is much more difficult and leaves longer-term consequences."

The Buruli ulcer on Jacinta Mazzarella's ankle was the size of a 50-cent coin before it started to heal. ( ABC News: James Oaten )

Infections contracted in backyards

To combat a surge in cases, the CSIRO has teamed up with Victorian government agencies, hospitals in Geelong and Melbourne, and the University of Melbourne to try to solve the mysteries surrounding the ulcer.

Over a two-year period, scientists plan to visit 120 homes of people who have caught the disease, as well as 120 homes not linked to infections.

They are also collecting soil, water, possum faeces and mosquitoes for testing.

"It's suspected that a lot of people actually catch the disease in their own garden," the CSIRO's Kim Blasdell said.

It is thought mosquitoes could be responsible for spreading the flesh-eating bacteria.

Contaminated soil or water that infects a cut or a scratch could be another cause.

Animals including possums, bandicoots, koalas and even domestic cats and dogs, are suspected to act as a carrier or "reservoir" of the disease.

"The idea behind this work is to try and get a much clearer understanding of how people are catching the bacteria," Dr Blasdell said.

The CSIRO is screening mosquitoes for Mycobacterium ulcerans. ( ABC News: Joanna Crothers )

"People are suffering with this disease for a long period of time and at the moment we don't know how to tell people how they can protect themselves."

In Victoria, the ulcer is colloquially called the Bairnsdale ulcer, because of an outbreak in the East Gippsland town in the 1930s.

In Queensland, it's known as the Daintree ulcer.

The northern state experienced a spike of 54 cases in 2011, but infections have sharply declined since to just a handful of infections.

But Victoria has had more than 330 cases diagnosed this year alone.

The most affected areas include the Mornington Peninsula (146 cases), Frankston (14), Greater Geelong (23) and Bayside Melbourne (14).

But increasing number of cases being diagnosed in new areas, including Melbourne's inner suburbs, is a concern for researchers.

"We don't really understand why [it's in] Victoria because for the rest of the world it's in the tropical areas," Dr O'Brien said.

Doctors initially did not know what was on Zhoe Jess's arm. ( ABC News: James Oaten )

Ulcers spreading to inner-city areas

Communications worker Zhoe Jess lives in Melbourne's inner west, and contracted the disease so severely that she was at risk of requiring a skin graft.

She has no idea how she became infected.

"I wasn't in any of the hotspots," Ms Jess said.

After her disease evolved from a small lump to an open wound, the 42-year-old had a range of medical tests.

Zhoe Jess was warned that her two ulcers could join, requiring a skin graft. ( Supplied )

She also underwent surgery to remove what the doctor thought was an abscess.

It was only after that surgery that Ms Jess suggested that she might be infected with a flesh-eating disease.

"It's spreading around and that scares me a lot," she said.

Ms Jess's infection appears to be dying after a strong concoction of antibiotics that often leaves her feeling sick and exhausted.

"I absolutely know there are people out there with this that are far worse off than I am," Ms Jess said.

"What petrifies me now and going forward is other people getting it. I can't imagine a child or an elderly person going through this."

Doctors now claim a 99.5 per cent success rate in treating the infection.

But people are being urged to take precautions this summer.

This includes being mosquito safe, removing stagnant water that can act as a breeding ground for mosquitoes, cleaning and protecting cuts and scratches, and wearing gloves when gardening.