Avoid enclosed, highly populated places where you could come in close contact with each other. Also wash your hands, wear a mask, and ventilate the room. These steps for infection prevention must be adhered to completely.
"Avoid enclosed, highly populated places"=「密閉、密集した場所を避けて」
"where you could come in close contact with each other"=「お互い密接な接触ができるような場所」
"Also wash your hands, wear a mask, and ventilate the room."=「そして手洗い、マスク着用と部屋の換気をするように。」
"These steps for infection prevention must be adhered to completely. "=「このような感染予防対策の手段は徹底的に守られなければいけません。」
We should avoid confined and enclosed spaces and wash our hands, and cover the mouth with masks to prevent the spread of the coronavirus.
「密閉密集密接を避け、手洗いマスク換気等の感染防止策を徹底」
"We should avoid confined and enclosed spaces and wash our hands, and cover the mouth with masks to prevent the spread of the coronavirus."
と言うことも出来ます。
"confined"は、「狭い・限定された」
"enclosed"は、「閉ざされた」
"prevent"は、「防止する」
"spread of the coronavirus"は、「コロナウイルスの蔓延」
という意味です。
ご参考になれば幸いです。