A health care worker at Los Angeles International Airport in California places a Covid-19 nasal swab test into a specimen bag on November 18.
A health care worker at Los Angeles International Airport in California places a Covid-19 nasal swab test into a specimen bag on November 18. Patrick Fallon/AFP/Getty Images

The US just surpassed a tragic milestone of more than 250,000 coronavirus deaths and more than 11.5 million infections nationwide, according to data from Johns Hopkins University.

Just how bad things could get will be determined by Thanksgiving celebrations next week. Health officials have warned against traditional indoor gatherings that seem to be a big driver in the surge of cases.

Each family will need to weigh the risks of celebrating Thanksgiving in person or virtually depending on individual circumstances. As well as taking precautions like masks and hand sanitizer, Dr. Sanjay Gupta has said the dose of the virus you receive might make the difference between being asymptomatic, getting mildly sick or becoming critically ill.

“It’s not because you need a certain number of particles of virus to infect a cell – it just increases the odds that one of those viral particles will make it into the cell and infect it, setting off the chain reaction,” he explains.

Another way to think of it is like conception: You don’t need millions of sperm to fertilize an egg – you only need one – but men make millions of sperm to improve the chances that one will reach the egg, overcome its defenses and fertilize it.

“Each person has a different amount of virus that they need,” explains Erin Bromage, an associate professor of biology at the University of Massachusetts, Dartmouth.

“Somebody that is immunosuppressed, or somebody that is stressed, for example, may need less of a [virus] challenge in order to get the same outcomes as somebody that is in a healthy condition,” she adds.

Put it all together, and the chance of infection depends on the physiology of the potential host, as well as their personal behaviors and health habits such as smoking status, diet, physical activity and sleep. An elderly or unhealthy host in the face of large, recurrent exposures is clearly the worst case scenario. But a medically fragile person could be sickened by even a low dose of virus; conversely, a healthy person can be overwhelmed with a high enough dose.

If you are planning on gathering with family or friends this holiday season, CNN’s Dr. Leana Wen explain some of the safest ways to do so in this video.

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