BBC News website age & disability correspondent
People with cancer, HIV and multiple sclerosis (MS) have new legal rights from today.
Patients have immediate protection after diagnosis
Such individuals are now protected by the Disability Discrimination Act (DDA) from the time of diagnosis.
It means that employers will not be able to sack someone because they have found out that they have one of the conditions.
People with mental health problems also have new rights - only some of whom enjoyed protection under the law.
Previously, a person had to prove that their mental health had a "substantial and long-term impact" on their life, and that the condition was "clinically well recognised".
Mental health service users had found the need to make their condition conform to specified diagnoses often left them without legal protection.
Welcoming the legal changes, the chairman of the Disability Rights Commission (DRC), Bert Massie, said a significant loophole in the law had now been closed.
"We've been unable to help people who've been diagnosed with serious illnesses and then treated unfairly because they didn't fall under the legal definition of disability," he said.
"This was plainly wrong - people with serious, long-term conditions shouldn't be discriminated against, full stop."
Other groups have also responded positively to the changes.
"Breast cancer patients will warmly welcome this landmark new protection against unfair treatment," said Anna Wood of Breast Cancer Care.
She said many families come under immense financial pressure because of the extra cost of having treatment.
"The added fear of losing a job, being overlooked or unsupported by employers, and being turned away from using everyday services can make an extremely difficult time even worse."
Ms Wood said the challenge now was to make sure that people with breast cancer know that this protection exists.
The National AIDS Trust (NAT) says employers should get up to speed on the new laws.
Previously, only people experiencing symptoms of HIV were protected by law.
But the NAT says that discrimination against people with HIV often occurs directly after the diagnosis because of the fear and misunderstanding surrounding the way the virus is transmitted.
"People living with HIV have a legal right to confidentiality and to work without fear of dismissal," said NAT chief executive, Deborah Jack.
"Yet HIV positive people are being failed by their employers."
The laws which have just come into effect are the first parts of the 2005 Disability Discrimination Act to be implemented.
Private clubs with more than 25 members have also been brought within the scope of the law, as have the publishers of discriminatory advertisements.