This Jan. 21, 2016 photo shows the water tower at the Flint, Mich., water plant. (Perry Rech/American Red Cross via AP)

As the contaminated water crisis unfolds in Flint, Mich.,  each wrinkle seems to expose a fresh horror. The high levels of metal in the water have received national attention, with President Obama calling the corrosion of lead plumbing “inexcusable” on Thursday. In the glare of continued public scrutiny, local experts have also brought to light possible biological contamination. Don Kooy, president of Flint’s McLaren Regional Medical Center, revealed early Sunday that the hospital had traced a Legionnaires’ disease outbreak to its water supply more than a year ago.

Is the long delay between this discovery of a potential source of harm and a public announcement just another facet of government incompetence in Flint? If the Flint officials who knew about the disease — a Centers for Disease Control and Prevention pathologist recommended a state investigation in April — will join those who have already resigned remains to be seen. But when it comes to Legionnaires’ disease, uncertainty isn’t unusual. In fact, it’s something of a defining characteristic.

Of the 87 cases of the disease reported in the area since June, Kooy thinks that two may have been linked to drinking the hospital’s water. What no one knows for certain is where the Legionnaires’ bacteria originated. “We were concerned that the city water was the source of it,” he told the Associated Press, “but to this day, I don’t think we could make a definitive statement.”

Take a look at the key moments that led up to Flint, a city of 90,000, getting stuck with contaminated water. (Claritza Jimenez/The Washington Post)

Amy Pruden, an expert in environmental microbiology at Virginia Tech, told The Washington Post the only way to find the “smoking gun” is to isolate the bacteria from patients and the water to look for a match. The problem is that the bacteria are notoriously tricky to grow in the lab, having what Pruden calls a Goldilocks-like preference for “just right” conditions.

Asked whether laboratory isolation should have been attempted in Flint, she said: “I’m not clear on the legal regulations,” she said, “but it’s definitely good practice.”

Legionnaires’ disease is a relative newcomer to the medical plague pantheon, having infamously claimed 25 lives in the first known outbreak in 1976. Simply because it is new, however, does not mean it is a particularly unusual or exotic malady. When our macrophages — cells in the immune system — encounter the bacteria Legionella pneumophila, which look like microscopic grains of rice sporting curlicue tails, the white blood cells do what they’re meant to — they ingest the invaders. But the Legionella bacteria don’t die, exploding out of the cells. When the infection floods the lungs, the result is severe pneumonia.

Even when this microscopic insurrection is not lethal, it can have long-term effects. Writing at the nonprofit Legionella.org, a Michigan woman named Kim (no last name given), shares the story of her mother who fell sick in June. Puzzling email correspondence foreshadowed her mother’s diarrhea and kidney failure; 11 arduous days in the hospital saved her life. Still, there’s a long road to recovery. “It seems that my mom has aged 20 years in the past month,” Kim wrote.

Although about 8,000 to 18,000 people are hospitalized with the disease each year, we can’t transmit Legionnaires’ from person to person. The classic mode of transmission is bacteria inhaled from contaminated water mist, which happened with the New York cluster traced to Bronx cooling towers. But in Flint — where the outbreak had a higher per-person incidence than in New York — it was likely drinking the water, not inhaling it, that led to infection.

Crack open any given water pipe, and you could see a slimy layer of biofilm coating the insides, Pruden pointed out. “It’s not necessary that the bacteria were in the Flint River — small amounts can be in the biofilm, and if the water chemistry changes, then they’ll start to multiply.”

Pruden, along with her environmental and civil engineering colleagues at Virginia Tech, have been conducting an independent assessment of Flint’s water supply. Their research shows the switch from treated Detroit water to Flint River water, with high levels of chloride, precipitated the crisis. “The massive corrosion that ensued with the Flint River water — besides destroying the pipes — ate up chlorine,” she said.

Chlorine concentrations in public water aren’t as high as the levels in your local YMCA pool, but the chemical has the same purpose: to kill biological contaminants. When the Virginia Tech researchers attempted to measure chlorine levels at 17 locations in Flint, they came up empty-handed in seven places. “The farther the water traveled through the pipes, the less likely we were able to detect any chlorine,” Pruden said. “That’s probably the biggest thing that made this system susceptible.”

That biologists still don’t have a clear picture of Legionnaires’ disease, which sadly, won’t do much to curb the frustration and outrage felt in Flint. “I’m pretty angry,” Terri Nelson, whose husband fell ill from the disease and died after staying at McLaren hospital, told NBC News on Thursday. “I’d like someone to be accountable.”

Whether state officials had a legal obligation to make an announcement is almost beside the point, in Pruden’s view. “If Flint has taught us anything,” she said, “the more transparency the better. If they had reported it, it would have been another line of evidence.”

Ben Guarino is a freelance journalist.