Evidence-based explanations of the coronavirus crisis, from how it started to how it might end to how to protect yourself and others.

To combat the growing coronavirus pandemic, US cities and states are beginning to impose strict measures meant to encourage social distancing and to limit the spread of infection.

Friday, New York and Illinois joined a number of other states, including California and New Jersey, in asking residents to go outside only for necessities. The requests mean by the end of the weekend, more than 70 million people — around one in five Americans — will be under orders to stay home.

“No, this is not life as usual,” New York Gov. Andrew Cuomo said Friday. “Accept it and realize it and deal with it.”

Commonly referred to as “shelter in place” orders, the measures vary slightly by state. However, they largely bar people from leaving home except for exercise and for essential travel: to and from work, hospitals, or the grocery store. The orders can be enforced by law enforcement, but it’s not clear how frequently police will cite people who violate the orders.

Police in San Jose, California, for instance, have said they currently are trying to educate those they observe refusing to comply — a stance the city’s police chief said will evolve: “Our current plan is to move from an educational stance to an enforcement stance on any business that has been contacted, warned, and then reopens,” Chief Eddie Garcia said Friday.

Other officials have taken a sterner stance, like Cuomo, who said Friday, “These provisions will be enforced ... These are not helpful hints.”

States that do try to enforce the orders face a numerical challenge, however. Wendy Mariner, a professor of health law at Boston University School of Public Health, told Vox: There are far fewer police than there are citizens. “All large-scale measures present obstacles to enforcement,” Mariner said.

Other states are not yet mandating residents not leave home, instead closing restaurants, bars, gyms, and other businesses as an early attempt to stop people from coming into contact with one another.

Here are the rules in some of the US’s largest and most affected states.

California

The most populous state in the US, encompassing 40 million people, went under a “stay at home” order Thursday, March 19.

Everyone must stay at home except for critical reasons, such as grocery shopping, visiting the doctor in case of emergency, or to exercise.

Gas stations, pharmacies, grocery stores, convenience stores, takeout and delivery restaurants, banks, laundromats, and essential government functions remain open.

Bars, dine-in restaurants, gyms, hair and nail salons, entertainment venues, and any public events or gatherings are closed.

16 “critical infrastructure” industries are allowed to continue operating but should work from home if they can.

New York

New York’s policy goes into effect at 8 pm ET on Sunday, March 22. This order and others put into place in recent days affect 20 million New York residents.

All businesses are closed except banks, hospitals, pharmacies, grocery stores, manufacturing plants, news media, and other “essential” services.

Gatherings “of any size for any reason” are banned.

Any people who are in public or doing essential work outside the home must stay at least six feet apart from one another.

People over the age of 70 and immunocompromised people must remain inside unless exercising, wear a mask around others, and pre-screen visitors by taking their temperature and checking for flu-like symptoms.

Barbershops, hair and nail salons, tattoo and piercing parlors, hair removal services, casinos, fitness centers, theaters, malls, bowling alleys, and amusement parks are closed.

Only takeout and delivery are allowed at bars and restaurants.

Public transportation and roadways aren’t shut down — though people are strongly discouraged from traveling if they can avoid it.

Illinois

Beginning at 5 pm CT Saturday, March 21, all of Illinois’ 13 million people are under a stay at home order.

People can leave their homes only to exercise and walk pets, to visit the grocery store, pharmacy, or gas station, or to attend work in essential industries.

All businesses and “places of public amusement” (like movie theaters, museums, zoos, and concert halls) are closed, except essential services such as health care, sanitation, airports, law enforcement, media, laundromats, and others. Playgrounds are closed.

All gatherings outside the home are prohibited and people who leave their homes should maintain six feet of distance from others.

New Jersey

Gov. Phil Murphy ordered the state’s 9 million residents to remain home Saturday, March 21, as confirmed cases climb above 1,000. According to reporting by the New Jersey Star-Ledger:

All nonessential businesses are to be shut down, excluding grocery stores and pharmacies.

All schools, casinos, dine-in restaurants, theaters, gyms, and malls are closed.

There’s a suggested curfew of 8 am to 5 pm and “nonessential travel” is strongly discouraged.

Those who ignore the governor’s order could face a disorderly conduct charge: “If folks are monkeying around, we will take action,” Murphy said Saturday.

Connecticut

The state of 3.5 million people has nearly 200 confirmed cases, and Gov. Ned Lamont issued a stay at home order on Friday. Beginning Monday, March 23 at 8 pm:

Nonessential businesses will be closed. Essential services such as grocery stores, healthcare, pharmacies, and gas stations remain open.

Public gatherings of any size aren’t allowed and people should limit outdoor activities, public transportation use, and other nonessential visits to businesses.

Sick people should only leave their home to get necessary medical care.

Washington

Washington, which has been the focal point of early confirmed coronavirus cases in the US, has more than 1,300 confirmed cases and 74 deaths. The state is not yet under a stay at home order, but Gov. Jay Inslee has said he may impose more restrictive measures soon because “we still are seeing people behaving as if this virus was not a mortal threat to people in this state,” NPR affiliate KUOW reported. Right now:

Schools statewide are closed until at least April 24.

All entertainment and recreational centers, bars and restaurants, and gyms are closed. Bars and restaurants are open for takeout.

Gatherings of more than 50 people are banned statewide and people are urged to stay home.

Nonessential businesses remain open, though workers are urged to work from home if they can.

Florida

The state of 21 million has more than 500 confirmed cases and 9 deaths to date. There’s no statewide shelter in place policy; Gov. Ron DeSantis has said he believes local measures are more effective. “If you go too hard, then I think people lose confidence and they rebel against it,” he said, according to the Sun Sentinel.

All restaurants and bars are closed statewide.

Concerts and other entertainment venues are closed in Broward and Palm Beach counties.

Beaches, parks, bars, and restaurants are shut down in Miami-Dade County.

All hotels must close by Monday night in the city of Miami Beach.

Texas

The nation’s second-most populous state, with nearly 29 million people, faces restrictions that went into effect at midnight on Friday, March 20, and will last through April 3.

Social gatherings are limited to 10 people.

Bars, restaurants, and gyms are closed. Takeout is allowed.

Visiting nursing homes is barred “unless to provide critical assistance.”

Schools are closed statewide.

As Vox’s Dylan Scott has explained, officials hope these measures will “flatten the curve” — that is, reduce the rate of infections quickly enough to both drastically limit the number of new cases and ensure the US hospital system isn’t overwhelmed by people seeking treatment.

And as Vox’s Ezra Klein has explained, experts believe the best way to flatten the curve is to limit interpersonal contact:

Adam Kucharski, the mathematical epidemiologist, has a very simple calculation on this point. He calculates that if each Covid-19 case leads to 2.5 more infections over five days, then a single case leads to 244 more cases over the course of a month. But if social distancing measures cut that number in half to 1.25 new infections for every case, that’s only four new cases over the course of a month. This kind of exponential math is really unintuitive. This isn’t about you or even the people directly around you. It is also about the chain you start. If you infect somebody and they have connections, you kick off in that chain that could be very deadly.

How long the shelter-in-place orders will need to stay in effect isn’t yet clear. But experts believe social distancing will likely need to be practiced for some time — at least until a vaccine becomes available, something that could take up to 18 months.