As a respected doctor and professor on first-name terms with decision-makers in the healthcare world, Professor Paul Tambyah, 50, could have worked within the system to advance his views on healthcare.

And he has many.

He believes strongly that access to healthcare is a basic human right, and that it is a shame to have healthcare treated like a commodity, with some treatments available only to those who can afford them.

It gets to him that patients with HIV in Singapore can get the best diagnosis in the world here - but then cannot afford the medication, because it is not subsidised.

He has written to officials on healthcare issues and received only polite replies. Nothing changed.

The senior consultant for infectious diseases at National University Hospital (NUH), who is also professor of medicine at the National University of Singapore (NUS), could also have taken the social activist route as his mother did.

Mrs Leaena Tambyah, 78, has been a well-known advocate for the disabled with the Asian Women's Welfare Association for decades, and helped start Singapore's first school for children with multiple disabilities .

"She stalks ministers," he quips, as he tells how she spent 10 years writing to ministers and waylaying them at events, to persuade the Government that ambulances ferrying disabled people to hospital should not need to bid for a certificate of entitlement.

But Prof Tambyah wants to go beyond helping people downstream, to be involved in asking the upstream questions.

"I don't have the social network that my mother has, to go and stalk ministers, so the alternative was (as) Goh Chok Tong told Catherine Lim: if you want to criticise the Government, join an opposition party.

"So I took his advice," he says, with a gleam in his eye. Mr Goh is Emeritus Senior Minister and Ms Lim a fiction-writer and occasional political commentator.

Prof Tambyah is now running on the Singapore Democratic Party (SDP) ticket in Holland-Bukit Timah GRC. His teammates are party leader Chee Soon Juan, 53, Mr Sidek Mallek, 55, and Ms Chong Wai Fung, 45.

The son of a social worker mother and doctor father, he grew up in middle-class Bukit Timah, and went to school at St Andrew's and Raffles Institution. After medical school, he did his post-graduate studies at the University of Wisconsin in the United States, returning in 1999.

Years of observing the healthcare system in Singapore left him frustrated. He checked out opposition parties and liked the SDP for its social democratic ideals which resonated with him.

He knew that Dr Chee had been widely discredited following well-publicised run-ins with his former employer, NUS; his former political mentor Chiam See Tong; and his political opponents, the People's Action Party.

He contacted Dr Chee anyway, in 2008. They met, and over time Prof Tambyah accepted that the latter had made mistakes but had moved on. He describes Dr Chee today as enthusiastic and sincere.

The road from being a caring doctor to opposition politics was not always smooth.

The first time he attended an SDP event was in May 2008, for a screening of the political documentary One Nation Under Lee: "The police came in the front and I left through the back ."

His mother and sister "went ballistic" when they learnt he had joined the SDP. But as he got more involved, they became "resigned", if not supportive.

He also worried about being sacked. Although he could go into private practice, he enjoyed the teaching and research work at the university - indeed, his profile on the NUH website lists numerous teaching awards and a creditable list of publications.

In the end, though, his boss told him NUS believed in freedom of expression and had no restrictions on staff. That cleared the way with his other employer, NUH.

He gave his maiden political rally speech during the 2011 General Election at Woodlands Stadium in Sembawang GRC. A day later, he spoke at another SDP rally at Boat Quay. He was tipped as a possible candidate in the 2013 Punggol East by-election, but the SDP decided not to contest.

All that high-profile opposition involvement did not prevent him from getting tenure at NUS in 2013.

His wife, Siok Kuan, is a senior lecturer of marketing at NUS. They do not have children.

During the interview at the SDP office in Ang Mo Kio, Prof Tambyah comes across as serious-minded and voluble. His answers are candid. Asked if he had considered running in a single-member constituency (SMC), he answers the question behind the question: "Everyone is telling me - you should go to an SMC, then you don't get dragged down by Chee Soon Juan's reputation. But SMCs are not easy to win, because they are carved out for a reason.

"They're either very strong PAP candidates or they are demographically full of new citizens or very likely PAP voters."

On the campaign trail at Empress Road and then Adam Road food centres, he is affable and outgoing, approaching residents, engaging with them easily. The SDP team gets a relatively friendly response, with some asking for photographs.

But when approached for interviews and names, most decline.

Except Mr Maurice Wee, 60, a semi-retired magazine writer, who had read Prof Tambyah's profile and thought him a "very good" candidate after a brief chat. But Mr Wee is not a voter in the constituency.

Prof Tambyah also wants to speak up on education, but it is clear that it is the inequities and inefficiencies in the healthcare system that have propelled him into politics.

He recalls one patient, a Chinese-educated woman in her 70s. One son is disabled, another jobless. A daughter gives her $250 a month. The old woman supports herself as a dishwasher. She needed a hand operation, and Prof Tambyah told her about the many financial assistance schemes available. She told him: "I'm not going to beg."

Prof Tambyah says: "That's the Singaporean spirit. These people don't want handouts. But the system should be more fair and generous. The reality is, it's not cost-effective because she's now at the stage where you can intervene. If you wait too long, she's going to fall, she's going to get a head injury and she's going to end up staying three months in hospital. Then you're going to have to put her in a nursing home."

In the end, he says, it will cost the taxpayer a lot more money to look after her, than making it easy and practically free for her to have an operation early.

"So it's the cost-effectiveness as well which is hard to figure out."