Dangerous viruses are highly contagious and pathogenic, i.e. they reach the target organ quickly causing severe morphologic changes. The tools of virus pathogenicity involve the capsid or envelope proteins that mediate adsorption and penetration to susceptible cells. In addition, virus-coded non-structural proteins trigger the shut-off of the host cell proteosynthesis and/or inhibit the host cell mRNA formation. In many cases, the virus-coded enzymes determine the rate of virus DNA or RNA synthesis, the destruction of host cell organelles and the assembly of new virus particles. The mechanisms selecting strains of higher or lower virulence among the virion progeny, follow the basic principles of genetics and apply equally to agents and their hosts. The outcome depends on the portal of entry into the human body, the route of transmission, the virulence of the agent and of its resistance to environmental conditions. Immobilization of the host by an average air-borne respiratory virus would create disadvantage for the spread of the given agent. Unless extremely contagious, it needs time to replicate and disseminates before killing the host. On the other hand, arboviruses transmitted by vectors may immobilize the host without influencing the frequency of transmission. The emerging viruses have not occurred before (HIV/AIDS, coronavirus/SARS) or they escaped attention, when affecting small and remote communities (Ebola and Lassa viruses). Some viruses, though certainly occurring before, had not been recognized to cause disease (hantavirus pulmonary syndrome, hepatitis C virus). These agents do not emerge as a result of mutations or recombinations, but rather represent a new germ, which survived in an animal reservoir. Re-emerging infections are diseases that once were a major public health problem, later on declined, but recently have become important again (reappearance of new influenza strains, dengue fever, rabies, West Nile virus and others). The control of emerging infectious agents by inhibiting the spread of particularly virulent variants of pathogens needs special control programs and surveillance systems (i.e. ProMED, Eurosurveillance). In addition, effective measures for fighting poverty and malnutrition, special vaccination and education programs, available medical care and treatment, and last but not least, alternative molecular methods for DNA or RNA identification (16S rRNA sequencing, broad range PCR and RT-PCR, representational difference analysis, novel toxin bioassays, comprehensive host gene expression profile etc.) are desirable. Increased urbanization and population density, social and political factors, local armed conflicts, increased migration and travel, natural and environmental changes and unexpected climate disasters make outbreaks of new infections possible and unpredictable.