sub-Saharan Africa, according to a report by mobile industry association GSMA and global consultants PwC released at the February 25-28 Mobile World Congress in Barcelona.
Africa's population of one billion is among the world's least "connected" yet more than a third already own a mobile phone, often in the most isolated areas, added a study by Wireless Intelligence.
The same is true in other parts of the world.
In India, a lot of people have no access to Internet, they sometimes can't read, but they have a mobile phone," said Sara Chamberlain, head of information and communications technology for India for BBC Media Action.
"It's often the only device they can access," said Chamberlain, who works on mobile health campaigns.
An SMS text information campaign can significantly improve the impact and efficiency of organisations' disease prevention programmes, she said.
"In maternal, newborn and child health, we are witnessing the game-changing effects that mobile technology has to offer through services such as pregnancy and birth registries, immunisation and nutrition tracking," said Patricia Mechael, executive director of mHealth Alliance.
These services could prove useful in sub-Saharan Africa for instance, where more than 1.2 million newborns die each year and one in nine children do not reach the age of five, according to the report by GSMA and PwC.
But the financing of mobile health in the developing world is still uncertain.
"The question of financial sustainability and ultimately 'who pays?' poses persistent challenges," said a report by mHealth Alliance and Vital Wave Consulting, also published at the World Mobile Congress in Spain.
In richer countries, meanwhile, money is pouring into "mobile health".
One of the main aims is to let people with chronic illnesses better manage their conditions day-to-day and reduce time spent in hospital.
According to PwC, by 2017 mobile health services could save $400 billion (300 billion euros) a year out of a total annual health expenditure of $6 trillion in the leading industrialised nations of the OECD.
"The direct benefit of this is reduced visits to healthcare practitioners, reduced costs and fewer expensive drugs," Victor Higgs, managing director of US firm Applied Nanodetectors, said in the report.
Several smartphones now offer applications that link with diagnostic tools, for example to perform urine analysis or check the blood sugar levels of people with diabetes.
Ucheck, available for Apple's iPhone from March, can check for 10 different substances by analysing a photo of a standard urine test strip.
GlucoDock, which connects to an iPhone, allows diabetics to check their blood sugar levels before and after eating.
Israeli firm Aerotel Medical Systems showed off its Mobile-CliniQ application, which works with Android-powered phones and links via a wireless Bluetooth connection to a device that can check blood pressure, pulse, cholesterol levels and a patient's weight, all of which can be relayed to medical staff.
For those who want to lose weight, former French cyclist Erwann Mentheour has joined with Data Addict to launch Fitnext, which allows users to count calories by selecting photos of various foods and discover how much effort it would take to lose them.