The novel coronavirus is racing through Indiana's nursing homes at a disturbing pace and with deadly consequences.

At least 120 people have died from COVID-19 at Indiana's long-term care facilities. That's according to what had been reported to state health officials as of Wednesday.

It's a testament to just how devastating the virus can be in nursing homes and other senior living centers. Just five days earlier, only 46 deaths had been reported.

That's a 260% increase in less than a week. And it represents 27% of the state's total deaths.

Other states are seeing high death tolls at nursing homes, too. USA TODAY found at least 2,300 nursing home deaths across the country.

Here are five reasons the virus has been so lethal in Indiana's nursing homes.

1. Seniors are more likely to die

Because COVID-19 is a new disease, information about risk factors is somewhat limited. But one thing has become clear: It is especially deadly for seniors.

In Indiana, Hoosiers age 60 and older accounted for nearly 37% of those who have tested positive for the virus, but they made up 88.7% of the state's 477 deaths, according to data reported to the Indiana State Department of Health as of Thursday.

That's roughly in line with what's happening nationally. The CDC says 8 out of 10 deaths reported in the United States have been in adults 65 years old and older.

That demographic makes up the vast majority of nursing home residents.

2. People with existing health problems get sicker

People with underlying medical conditions are also at higher risk of becoming very sick, according to the CDC.

That includes those with chronic lung disease, moderate to severe asthma, serious heart conditions, diabetes and those with conditions that compromise their immune system.

Such underlying medical conditions are the very reason many people seek nursing home care.

3. Group living facilitates spread

The virus spreads like wildfire in congregate living environments, where residents share hallways, bathrooms and dining areas. This makes places such as nursing homes and prisons especially vulnerable.

Nursing homes have tried to control the spread of the disease by isolating residents in their rooms, eliminating group social activities, and serving food to them in their rooms, rather than in communal dining areas.

4. A lack of tests makes response difficult

Testing for the virus has been limited across the country because of supply shortages, but this has been especially true in Indiana, which has lagged most other states in per capita testing.

Until recently, state health officials were only recommending testing for nursing home residents who were showing symptoms. That meant that even when a positive case was identified, not everyone was getting tested. This left nursing homes largely blind as they worked to contain outbreaks, making it difficult to segregate residents who had the virus from those who did not.

Only within the last week or so have state health officials encouraged more liberal testing. By then, however, it was too late for many homes.

5. Low staffing makes homes more susceptible

Experts say low staffing levels can also make homes more susceptible to outbreaks and disease spread. That's particularly troubling in Indiana. Its nursing homes rank 49th in the nation for total nursing staff hours.

Insufficient staffing and low pay can put pressure on employees to come to work sick. Basic infection controls such as hand washing and sanitizing surfaces can be overlooked. It's more difficult to prevent residents from congregating. And workers are more likely to work at multiple facilities, potentially spreading the virus.

Staffing shortages have only grown more pronounced amid the coronavirus pandemic. Many workers have become sick and some have quit amid safety concerns.

Contact IndyStar reporter Tony Cook at 317-444-6081 or tony.cook@indystar.com. Follow him on Twitter: @IndyStarTony.

All of IndyStar's coverage of coronavirus is being provided for free to our readers. Please consider supporting local journalism by subscribing to IndyStar at indystar.com/subscribe.