Flood waters can leave behind a variety of hazards for survivors, from infectious diseases, to diabetes care, to infections, to even wild animals.

The damage left by Hurricane Harvey is just starting to be understood with thousands still in shelters and an unknown number of houses underwater.

But the flood waters have finally started to recede in Houston, days after it was hit by the brunt of the storm and driving rains.

Past disasters like Hurricane Katrina in New Orleans have revealed that once the storm is over, the cleanup and recovery process can still be dangerous.

Dr. Richard Besser, president and chief executive officer of the Robert Wood Johnson Foundation, said there are number of health risks people face when they finally return home and start to rebuild.

“When people see pictures of the hurricane and the storm, what comes to mind are the immediate dangers that can be caused by the wind and water,” Besser told Healthline. “It’s important to recognize that the period of cleanup poses very significant health risks in the long term.”

With much of Houston still flooded, first responders and medical staff have focused on helping those in acute distress.

But health experts point out that people with chronic conditions could be running out of time, even if they aren’t seriously ill.

Dr. E. Anne Peterson, the senior vice president of Global Programs at the relief and development organization Americares, said their teams are concerned about people like those with diabetes or in need of dialysis who could be cut off from care.

“You have people who have chronic diseases, or in other cases pregnancy, and are in need of immediate acute care,” Peterson told Healthline.

She said they have already been working with partners to help evacuate people who need dialysis so that they can get care at a dialysis center that is open.

“The access to care will become increasingly life-threatening and serious as they are cut off from things,” Peterson said.

In addition to affecting those who need care now, a storm this size can potentially disrupt people’s access to their doctors.

Besser said that public health experts learned from Hurricane Katrina in 2005 that many people who left after the storm never came back to the area.

As a result, people with chronic or acute health conditions were not able to continue treatment with their normal physicians or get access to other health resources that would have been made available to them as hurricane survivors.

Besser said his team has been working with the American Red Cross to track patients and ensure they don’t fall through the cracks.

He said going forward, increased reliance on electronic records could help people who leave the area.

“When we think about the value of electronic medical records, so more people have that information electronically… It does allow people to get in touch,” with their doctor, he said.

Health officials warn that people may return to their homes after the storm only to find new occupants: wild animals.

“One of the things that people don’t think about is that animals, snakes, and insects also try to hide from the flood, and they hide in the abandoned houses,” Peterson said. “As [people] clean up, things like snake bites and animal bites become more common.”

She advised people to be especially wary when cleaning out areas like cabinets or closets, where they can’t see clearly inside and not simply reach their hand in without being sure there isn’t anything hiding.

Additionally, she said that the Americares teams have been bringing in tetanus shots for people who may get open wounds while working in the flood waters.

“As people go to clean up, they get cleanup injuries,” Peterson said. If they cut or injure themselves, “they are very high risk for tetanus.”

The water itself, full of pollutants and gasoline from stranded cars, can be hazardous.

Besser pointed out that as people head back to fix their homes they’ll also need to try and stay clear of any standing water left behind.

“The water itself can pose dangers, from chemicals in that water,” he said. “You want to make sure you have proper protection.”

He also warned people to be careful when using generators for electricity.

“All generators produce carbon dioxide,” he said. “It is odorless and clear, and people don’t recognize that they are being exposed.”

He said the generators should be away from homes and never used indoors.

“The fumes are deadly,” he said.

Dr. William Schaffner, an infectious disease expert at Vanderbilt University Medical Center, said that crowded shelters full of people exposed to dirty water in the floods can be prime conditions for certain disease outbreaks.

“In those congregate settings, where people are close to each other for a prolonged period of time it’s a good way for infectious diseases to spread,” he explained. “Respiratory viruses spread readily… it might be a setting where you get some flu.”

In addition to respiratory viruses, he said he would be concerned about a virus like the norovirus — often called the stomach flu — that can spread quickly through a population.

“We did have one outbreak of norovirus,” during Hurricane Katrina, he said. “It’s so readily transmitted.”

If the storm has disrupted water lines, it can mean people are left without clean drinking water. As a result, the risk of waterborne disease outbreaks can increase.

Peterson said they have also been focused on making sure that people in Houston have access to clean drinking water and a way to dispose of wastewater so none of these illnesses are easily spread.

“It isn’t very fancy, but having drinking water can make a huge difference,” she said.

With pools of standing water likely to remain even after the flood subsides, Schaffner said that mosquito-borne illness will likely be a concern for public health officials.

“The three that I would be concerned about are West Nile, our issue from last summer the Zika virus… and then there are a group of viruses that we could call the encephalitis viruses,” which can cause brain swelling, Schaffner explained. The diseases “could in a circumstance like this be spread by mosquitoes.”

As people get to the difficult task of rebuilding their lives, they will face a daunting task that can be mentally and physically taxing.

“When we think about the big issues that drive people’s health, it’s about rebuilding communities,” he said.

He pointed out people will be cut off from work or services to take them to work and need help with food and shelter as they rebuild.

Besser said that as recovery grinds on, people will become at risk for a host of mental health issues related to the trauma that they experienced.

Besser explained it’s key that people understand feelings of anxiety, depression, or other symptoms of post-traumatic stress disorder are natural, and that officials ensure there is a way these people can seek help.

“Once the first responders go away, the public interest in the disaster and rebuilding often goes away as well,” he said. “That’s when you can see mental health [issues] and depression.”