Figure 31-4. A tuberculoma of the pons on a gadolinium-enhanced MRI (left panel). There is a thick, uniform enhancing rim. The mass behaved clinically like a malignant brain tumor. The right panel shows the same lesion after antituberculous treatment.
Figure 31-5. Diagram of the evolution of neurosyphilis.
Figure 31-6. MRI showing a rim-enhancing Toxoplasma abscess in the deep left cerebral hemisphere in a patient with HIV. There is mass effect and surrounding edema, features which are variable.
Figure 31-7. Cysticercosis on CT without contrast. Multiple vesicular lesions, some with visible scolices and without mass effect (left panel) that became calcified 2 years later (right panel).
Figure 31-8. MRI of schistosomiasis (S. mansoni) with gadolinium enhancement of patchy lesions in the left temporal lobe in a traveler returning from Ghana.
Chapter  31    Bacterial, Fungal, Spirochetal, and Parasitic Infections of the Nervous System
Viral Infections of the Nervous System
A number of viruses share the unique tendency to primarily affect the human nervous system. In some conditions, the systemic effects of the viral infection are negligible; it is the neurologic disorder that brings them to medical attention, that is, the viruses are neurotropic. Included in this group are the human immunodeficiency viruses (HIV-1 and HIV-2), the group of human herpes viruses including herpes simplex viruses (HSV-1 and HSV-2), herpes zoster or varicella zoster virus (VZV), Epstein-Barr virus (EBV), cytomegalovirus (CMV), poliovirus, rabies, and several seasonal arthropod-borne viruses (Flaviviruses). Some of these exhibit an affinity for certain types of neurons: for example, poliomyelitis viruses and motor neurons, VZV and peripheral sensory neurons, and rabies virus and brainstem neurons. Yet others attack nonneuronal supporting glial cells; John Cunningham (JC) virus causing progressive multifocal leukoencephalopathy is the prime example. For many of the rest, the affinity is less selective in that all elements of the nervous system are involved. Herpes simplex, for example, may devastate the medial parts of the temporal lobes, destroying neurons, glia cells, myelinated nerve fibers, and blood vessels; and HIV may induce multiple foci of tissue necrosis throughout the cerebrum. These relationships and many others, which are the subject of this chapter, are of wide interest in medicine.
Pathways of Infection
Viruses gain entrance to the body by one of several ways. Mumps, measles, and VZV enter via the respiratory passages. Polioviruses and other enteroviruses enter by the oral–intestinal route, and HSV enters mainly via the oral or genital mucosal route. Other viruses are acquired by inoculation, as a result of the bites of animals (e.g., rabies), ticks, mites, or mosquitoes (arthropod-borne or arbovirus infections). The fetus may be infected transplacentally by rubella virus, CMV, and HIV. In all these cases, viremia is an intermediate step to seeding the brain or cerebrospinal fluid (CSF).