Wales' Health Minister Vaughan Gething is self-isolating for 14 days after his son developed a cough.
He was due to be making a statement on the response to coronavirus in the Senedd on Tuesday.
In a Twitter video, he said he will not be in the assembly "for the forseeable".
It comes after four other assembly members said they are in self-isolation, bringing the total to five.
Plaid Cymru's North Wales AM Llyr Gruffydd is also-self isolating.
Mr Gruffydd, in a tweet, said: "One of my children has showed covid-19 symptoms so I'm following advice and self-isolating for 14 days.
"I'll be working from home as best I can."
The government has advised people in households where a person has developed a new cough or high temperature to self-isolate for 14 days.
Meanwhile the Welsh Assembly's Senedd building has closed to all-visitors "with immediate effect".
All non-time-critical proceedings are to be suspended "for the time being", the body that runs the legislature, the Assembly Commission, said in a statement.
"The decision to close the Senedd to all visitors has been taken in order to protect the public, staff and members in line with latest guidelines issued by Welsh Government and UK Government," it said.
Minister for Health and Social Services @vaughangething is self-isolating for 14 days as his son is not well, in line with our advice.— Welsh Government (@WelshGovernment) March 17, 2020
The Minister is continuing to lead the #NHSWales response from home. #CoronavirusUK pic.twitter.com/JChSzhwUaq
In a video Mr Gething said he fell into the category of people requiring self-isolation "as my son has a new wheezy cough".
"So I'm not going to be in the assembly for the forseeable.
"I'm following the advice we're asking the public to follow as well."
He intends to speak to AMs later via video link.
Those also self-isolating are Plaid Cymru leader Adam Price, Brexit Party group leader Mark Reckless and Neil McEvoy.
Everyone has been told to practice social distancing and those over 70 have been advised to take extra care.
This has affected AMs, who are working from home: Culture deputy minister Dafydd Elis-Thomas, who is 73; leader of the house Jane Hutt, who is 70, the children's deputy minister Julie Morgan, who is 75, Welsh Health Minister Gething in self-isolation and Tory South Wales East AM Mohammad Asghar, 74.