If not treated in some form, do most who acquire malaria die? What about primates, such as orangutans that live in the wild and would not be treated as such. Do they die or do they become chronically ill within period of remission?
That’s a really good question, and the answer is: it depends! In humans, the most deadly form of malaria is Plasmodium falciparum—when infected for the first time, if not given prompt treatment, many people will die from this infection. However, after repeated infections, people develop acquired immunity to the P. falciparum parasite which means they are increasingly able to survive subsequent infections without treatment. This reason of acquired immunity is why young children, who do not yet have immunity, and visitors to malarial areas tend to have the most severe infections and most require treatment in order to survive.
The other three major forms of human malaria, P. vivax, P. malariae and P. ovale, are generally less deadly, though they can also result in death in some circumstances if the person does not have immunity and is not treated. Although much less common than P. falciparum, P. knowlesi is the fifth type of malaria to infect humans (it is more commonly an infection of macaque monkeys in south-east Asia), and because it replicates in a 24-hour cycle (the other types of human malaria have either a 48 or 72 hour cycle), high parasite loads can establish very quickly, leading to severe disease. As such, P. knowlesi is also quite dangerous and a high proportion of untreated cases result in death.
It is great that you ask about malaria in other primate species—as with humans, some forms of malaria are tolerated reasonably well while others are more deadly. It varies depending on the type of malaria as well as the species of primate. So, for example, P. knowlesi in long-tailed macaques is rarely observed to cause severe disease. In fact, infected macaques sometimes don’t even appear to have any symptoms. In contrast, if rhesus macaques are experimentally infected with P. knowlesi (the transmission range of this type of malaria does not overlap with the natural range of rhesus macaques), almost 100% of them will die without treatment.
You ask specifically about orangutans: one problem is that it is unclear which, and how many, species of malaria infect these apes. Past research has uncovered two species which are thought to be unique to orangutans (namely P. silvaticum and P. pitheci) while molecular studies have also shown non-specific species, namely human P. vivax and macaque P. cynomolgi and P. inui. As such, while originally orangutan malaria was thought to be not very dangerous to these apes, more recently there have been reports of orangutans showing very human-like symptoms suggestive of more advanced disease. However, rarely do studies link symptoms and observations of parasites in the blood, so it is unclear which parasites are causing these symptoms, if indeed it is malaria at all (in some sanctuary/rehabilitation center settings, orangutans exhibiting malaria symptoms have responded positively to treatment with anti-malarials, though this is not definitive evidence that their symptoms were caused by malaria).
So, in short, more research should be done on wild primates, particularly using molecular tools, to ascertain accurately what species of malaria they are infected with, and whether they are associated with symptoms and/or severe disease.