Emerging viral diseases are new infections, while re-emerging diseases are previously controlled ones that return. They spread due to zoonotic transmission, mutations, globalization, and weak healthcare systems. Surveillance, vaccination, and global cooperation help control outbreaks.

Viral evolution occurs through mutations, recombination, and natural selection. High mutation rates, especially in RNA viruses, help them adapt to new hosts and evade immunity. Processes like antigenic drift and shift enable recurring infections. Understanding viral evolution helps in developing effective vaccines and treatments.

Viruses depend on host cells for replication and use host receptors to enter. They evade immunity by inhibiting interferon responses, disrupting antigen presentation, mutating surface proteins, or establishing latency. These strategies help viruses survive and cause disease.

Advances in viral genomics and metagenomics use high-throughput sequencing to identify and study known and novel viruses. These tools expand viral diversity knowledge, track evolution and outbreaks, and support better surveillance, vaccine development, and disease control.

Viral pathogenesis is the process by which viruses enter host cells, replicate, evade immunity, and cause disease. Damage results from cell destruction, immune suppression, excessive immune responses, or persistent infections. Understanding these mechanisms helps develop antivirals and vaccines.

Viruses are microscopic infectious agents made of DNA or RNA inside a protein capsid, sometimes with a lipid envelope. They cannot reproduce independently and must infect host cells, using their machinery to replicate and form new viruses, often damaging the host cell.