Malaria, the world's most widespread parasitic  disease, kills as many as three million people every year--almost all of whom are under five, very poor, and African. In most years, more than five hundred million cases of illness result from the disease, although exact numbers are difficult to assess because many people don't (or can't) seek care. It is not unusual for a family earning less than two hundred dollars a year to spend a quarter of its income on malaria treatment, and what they often get no longer works. In countries like Tanzania, Mozambique, and the Gambia, no family, village, hospital, or workplace can remain unaffected for long. Malaria starts suddenly, with violent chills, which are soon followed by an intense fever and, often, headaches. As the parasites multiply, they take over the entire body. Malaria parasites live by eating the red blood cells they infect. They can also attach themselves to blood vessels in the brain. If it doesn't kill you, malaria can happen again and again for years. The disease passed on to humans by female mosquitoes infected with one of four species of parasite. Together, the mosquito and the parasite are the most deadly couple in the history of the earth--and one of the most successful. Malaria has five thousand genes, and its ability to change rapidly to defend itself and resist new drugs has made it nearly impossible to control. Studies show that mosquitoes are passing on the virus more frequently, and there are more outbreaks in cities with large populations. Some of the disease's spread is due to global warming. For decades, the first-choice treatment for malaria parasites in Africa has been chloroquine, a chemical which is very cheap and easy to make. Unfortunately, in most parts of the world, malaria parasites have become resistant to it. Successful alternatives that help prevent resistance are already available, but they have been in short supply and are very expensive. If these drugs should fail, nobody knows what would come next. Which of the following may be the reason for the wide spread of the disease?
Its outbreaks in cities with large populations. Its ability to pass on the virus frequently. Its resistance to global warming. Its ability to defend itself and resist new drugs.