What happens next depends on the person's immune system. Pregnant women, children under five and those with tuberculosis or Aids are more vulnerable, while many inhabitants of endemic malarial regions carry the parasites without being ill, due to immunity acquired through repeated infection. In those who do succumb, symptoms appear within two weeks and vary according to the type of parasite. Generally, fever comes in cycles as the parasite moves to attack new blood cells. The infected person is also likely to suffer headache, vomiting and other symptoms that could be mistaken for a range of different illnesses. Plasmodium falciparum sufferers have 48-hour breaks between fever peaks that repeat themselves until the patient develops an immunity, the appropriate drug is administered or the person dies.