In a tweet, Eastleigh candidate Patricia Culligan claimed a Liberal Democrat standing in another seat "deliberately became HIV positive yet free NHS care v costly".
She was referring to Paul Childs who is standing in Liverpool Riverside. He revealed he was HIV positive last week.
In appearing to question the cost of treating British patients Ms Culligan went further than Nigel Farage, who has controversially said foreigners with HIV should not be treated for free by the NHS.
The UKIP leader's comments during last week's TV debate were greeted with heavy criticism from some of the other leaders. He insisted it was right to "put our own people first".
Patricia Culligan removed her tweet and apologised after UKIP was contacted by the BBC on Thursday evening.
In a statement, the party said: "We wholeheartedly support the NHS being free at the point of access and kept publicly-funded.
"We have spoken to Ms Culligan and advised her to retract the comment which seems to conflate two very separate issues."
In a follow-up tweet she said she "sincerely" apologised for "any totally unintended offence" , saying she had misread a Mail Online article about Mr Childs.
She added: "I have nothing but sympathy for sufferers."
It is not clear though if she believes the NHS should ration or reduce treatment of British HIV sufferers.
UKIP almost won the Hampshire seat of Eastleigh in a by-election in 2013.
Ms Culligan was selected last September to fight it for UKIP.
The Liberal Democrats said her comment was "disgusting and deeply offensive".
A party spokeswoman said: "The Liberal Democrats will always stand up for the liberal British values of tolerance, generosity and decency."
For a full list of the candidates in Eastleigh please click here.
For the corresponding list in Liverpool Riverside please click here.
UKIP's candidate in a key general election seat has been forced to apologise after she appeared to question the cost to the NHS of treating British people who are HIV positive.