"Immunity to infection varies between people," Roper explained to news.com.au. "For example, compare the immune level of a younger person versus an elderly person.
"Immunity can also vary according to a person's health status at any point in time. If a person is stressed — be that emotionally or physically — this can lead to reduced immunity and increased susceptibility to illness."
While many scientists are talking about underlying health conditions, obesity and old age being the main factors that make a person more susceptible, Roper says that even the healthiest among us could be suffering from a weakened immune system.
"Even elite athletes, if they start over-training, can stress their body and result in reduced immunity — despite being very fit for their sport."
The circumstances of your exposure
I caught Covid at the start of this year from my boyfriend, when we were isolating in a studio apartment, but I know lots of people who avoided the virus despite being in close proximity with infected people.
We've all heard of boyfriends who never caught it from their girlfriend even though they live together. Or 6-year-old boys who never passed it on to their sister or parents, despite them catching nearly every other cold under the sun from him.
So why are some people so lucky? Roper suggests it can have a lot to do with factors other than immunity.
"In households, there would also be other factors," she explained, "such as how much time one person spends in close proximity to an infected person in comparison with another.
"The size of the household would also have an impact on why there is more transmission in some homes compared with another. Two people living in a one-bedroom apartment is not the same as two people living in a three-bedroom house, and their opportunities for exposure will be different."
Prior infections
As it turns out, previous infections of any kind may have helped you escape Covid, according to research.
"Having a prior infection to another cold virus can confer some protection to Covid, or other respiratory viruses," said Roper.
"Exposure to other respiratory viruses can prime parts of the immune system, leading to better defence against infection by the Sars CoV-2 infection."
It's not a foolproof system, however. Roper notes that some virus can live in your body together — like influenza and Sars CoV-2.
Of course, there's also a good chance you did have Covid and just didn't realise it, according to Immunologist Professor Stuart Tangye.
"When we first started doing PCR testing, it was done on symptomatic people so we were obviously missing a lot of those asymptomatic people," Tangye told the ABC.
"I'm sure we missed a lot of positive cases over December and January too, where there was a supply and demand problem in terms of getting tests."
Genetics?
UK researchers this year performed the first human-challenge trial of its kind for Covid.
The study found 36 young and healthy people who had no evidence of previous Covid infection or vaccination. It exposed all of them to the virus, and only about half of them actually caught it — which was defined by two positive PCR tests in a row.
Of those who didn't catch Covid, about half of them did briefly show low levels of the virus. This suggests their immune systems shut it down pretty quickly.
"There's probably a few people who would have a really strong innate immune response [that] just quells the infection, without enabling the virus to get too far ahead," said Tangye.
"There are going to be people who are less susceptible to viral infection because they have differences in their genes, such as genes that are important for viral entry into your cells."
Although that group would be very small, Tangye also suggested this wasn't the first time some people have been found to have a genetic resistance to diseases.
"With HIV, for example, there is a very, very small number of people who are genetically resistant to infection," he said.
"That's because they have naturally occurring genetic mutations in a certain gene so the virus can't physically infect their T cells."
So maybe you’re genetically lucky, but more likely you were just lucky at the time of your exposure to Covid — or never realised when you had it.
