Charlie Jermyn died in May, about 30 hours after he was born in his parents' toilet in Penryn, Cornwall.
Microbiologist Dr James Gray told the inquest in Truro that Charlie had shown some signs of illness but he was not sent to hospital for treatment.
Sepsis happens when the body's immune system goes into overdrive.
More on this story and other news from Devon and Cornwall
Charlie's parents went to the Royal Cornwall Hospital in Truro on 8 May when Hayley Jermyn was heavily pregnant.
The couple were worried the baby would arrive quickly like one of their other children.
Mrs Jermyn was assessed by midwife Jane Calvin, who said the mum-to-be was "not in established labour" when she examined her five hours after arriving.
"She came in at 22:00 and by the time I got to examine her at 03:00, in all that time nothing had changed," she said.
Ms Calvin told the inquest there was "no sign of infection anywhere" and "I advised her to go home and get some rest".
Hours later Charlie was born at home but he died of sepsis on 10 May.
Dr Gray said: "If Charlie had been in hospital at a time when, or soon after, he first showed clinical signs of early-onset sepsis he would have received intravenous antibiotics at least 12 hours, and maybe more than 15 hours, before he died.
"In my opinion, he would, on balance of probability, have survived in such circumstances."
However, Dr Gray said he it was "impossible to say" what condition Charlie might have been left in if he had survived, for example he might have been left with brain damage.
He added that the form of Streptococcal infection Charlie had was "a very serious condition and has a high mortality rate".
The inquest continues.

The move is part of Police Scotland's cost-cutting plan to centralise control rooms.
If a decision is taken at a board meeting on 22 March it is thought likely that the centre will close quickly afterwards.
SPA chairman Andrew Flanagan said he had been consulting widely on the move.
He told BBC Scotland: "I think there are a number of local concerns about the closure. It's important we take account of local concerns.
"We need an assurance it's going to be effective."

Liverpool City Council used a online budget simulator to ask residents about a proposed council tax rise of 5%, plus a further 6% to pay for social care.
Joe Anderson said the plan would not go ahead after 57% of respondents said no.
He also revealed plans for a Liverpool lottery in his New Year message on his Liverpool Express blog.
He said following the "feedback... I will not be proposing to hold a referendum on any additional increase beyond the 4.99% limit set by government".
Work was continuing on the full details of the authority's budget, he said, which would then undergo consultation in 2017.
He added that he was "surprised and proud that so many, 43%, said yes".
"It's a truly heart-warming reminder of how caring our city really is," he said.
Green party leader Councillor Tom Crone said the consultation was a "sham exercise".
"The weak consultation was never going to secure support for a tax rise," he said.
He said the mayor's decision not to hold a referendum meant there would be "tens of millions of pounds more cuts to services in the coming years".
Liberal Democrat leader Councillor Richard Kemp, said: "The public were never going to buy it over concern about the waste of existing council tax income."
Mr Anderson said the Liverpool lottery card would help local causes, adding that more details of it would be revealed in the coming months.
"Liverpool people are amongst the most generous in the country and I believe will support the initiative to help us cope with the growing numbers of people dependent on us," he said.
Councillor Kemp said his party would oppose the lottery idea.
He said: "The overwhelming evidence is that existing lottery users are already from the most deprived parts of the community.
"A new Liverpool lottery will be a tax on the poor to pay for services to the poor."
A baby who was born in a toilet and later died of sepsis could have survived if he had been given antibiotics, an inquest has heard.
[NEXT_CONCEPT]
The chairman of the Scottish Police Authority has said Aberdeen's emergency control room will only be closed if it is safe to do so.
[NEXT_CONCEPT]
There will not be a referendum on an additional council tax rise in Liverpool after the results of an online poll, the city's mayor has said.