We have highlighted that the study of contact networks is fundamentally important to epidemiology and provides a wealth of tools for understanding and predicting the spread of a range of pathogens. As we have outlined above, many challenges still exist, but with growing interest in this highly interdisciplinary field and ever increasing sophistication in the mathematical, statistical and remote-sensing tools being used, these problems may soon be overcome. We conclude, therefore, that now is an exciting time for research into network epidemiology as many of the practical difficulties are surmounted and theoretical concepts are translated into results of applied importance in infection control and public health. Viral Encephalomyelitis  Encephalitic Viruses Can Use Neuronal or Non-Neuronal Pathways to Enter the CNS When a virus does invade the CNS, there are several routes by which infection of neurons can occur. The most common entry point is from the blood, and the level of viremia as a result of virus replication in peripheral organs often correlates with the likelihood of CNS infection. However, the blood-brain barrier (BBB), composed of vascular endothelial cells with tight junctions in contact with the foot processes of astrocytes, inhibits direct access to the brain parenchyma and neurons. Some neurotropic viruses can replicate in cerebrovascular endothelial cells, enter with infected leukocytes, or cross directly into the cerebrospinal fluid (CSF) through the porous capillaries of the choroid plexus. A specialized CNS entry pathway used by several viruses, most notably HSV, varicella zoster, and rabies viruses, is by way of nerve terminals in peripheral organs. These viruses can enter the nerve and then use neural transport mechanisms to transport the infecting virions to the neuronal cell body where replication occurs [6, 7] . A variation on this theme is infection through the exposed end processes of neurons in the nasal olfactory epithelium, followed by transport of the virus to the olfactory lobe within the CNS. Intranasal infection is commonly used to initiate infection of the CNS in experimental animals, but in humans this pathway may be important only in rare cases of aerosol exposure to a neurotropic virus [8] [9] [10] .