(CNN)The concept of herd immunity is a simple one. But achieving it? Not so much.
As the coronavirus pandemic spread throughout the world, doctors, scientists, and government leaders alike have said that once herd immunity was achieved, the spread of the virus would be less of a threat. Herd immunity is reached when the majority of a given population -- 70 to 90% -- becomes immune to an infectious disease, either because they have become infected and recovered, or through vaccination. When that happens, the disease is less likely to spread to people who aren't immune, because there just aren't enough infectious carriers to reach them.
There are just two ways to get there: widespread vaccination, which for Covid-19 is still many months away, or widespread infections that lead to immunity.
Herd immunity by infection is not without risks
Most doctors and experts agree that allowing Covid-19 to just plow through populations might help reach herd immunity more quickly, but that would also overwhelm hospitals. More people would die, not just from coronavirus but from other infections, too. That's why we're all stuck at home -- we're flattening the curve.
"The advantage of stretching out the number of cases is that we will not exceed the capacity of hospitals to care for those who are particularly sick," Dr. H. Cody Meissner, chief of pediatric infectious disease at Tufts University Medical School, told CNN's Michael Smerconish in March.
And then there's the issue that we don't really know how immunity works with this virus.
Dr. Maria Van Kerkhove, an infectious disease epidemiologist at the World Health Organization (WHO), said it's not known whether people who have been exposed to the virus become completely immune to it and if so, for how long. That's why governments should wait for a vaccine, she said.
The WHO has "seen some preliminary results, some preliminary studies, pre-published results, where some people will develop an immune response," Van Kerkhove said. "We don't know if that actually confers immunity, which means that they're totally protected."
A vaccine is a better answer, she added. "I mean, recently we had more than 130 developers, scientists, companies come together to say that they would be willing to work with us -- to work globally to advance a vaccine. And that is something that we will push and the whole world is waiting for."
And while, younger people are far less likely to die from Covid-19, they can still become sick enough to require hospitalization.
Even if catching Covid-19 once does make people immune to future infection, the US has not had enough cases to get close to widespread immunity.
"The level of people who've been infected, I don't expect it would rise to the level to give what we call herd immunity protection," Dr. Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, told CNN's Jim Scuitto on Tuesday.
"What it will mean, it would protect those who have been exposed, but at the community level there would not have been enough infections to really have enough umbrella of herd immunity," Fauci said.
As a vaccine has yet to be created for the novel coronavirus, some have argued that nations should forgo lockdowns entirely and try to achieve herd immunity by keeping the vulnerable indoors while allowing others to live out their normal lives -- and get infected.
UK's Prime Minister Boris Johnson seemed to support a similar belief back in March when he held off on banning large gatherings and closing schools.
However, Johnson later issued a nationwide stay-at-home order, effectively closing all nonessential businesses and banning public gatherings. He then contracted the virus himself, and spent three nights in the ICU. He has since been discharged from the hospital.
Why Sweden refuses to lock down
While much of Europe has gone into lockdown, one country has bucked the trend: Sweden.
Restaurants, schools and playgrounds in the Scandinavian country are open. Sweden's Foreign Minister Ann Linde has said it's not following the herd immunity theory, but rather relying on its citizens to voluntarily be responsible to prevent the spread of the coronavirus.