Final answer:
Viruses have genetic material and can evolve but lack cellular structure, metabolism, and the ability to reproduce without a host. The body protects against infections using white blood cells, physical barriers, and chemical secretions. Symptoms of some viral infections can reappear due to the virus's ability to remain latent and reactivate.
Step-by-step explanation:
Characteristics of Viruses
Viruses display some characteristics of living things, such as having genetic material and the ability to evolve. They, however, lack other key characteristics such as cellular structure, metabolism, and the ability to reproduce independently - relying entirely on a host cell for replication.
First Forms of Life
It is debated whether viruses could have been the first form of life on Earth. Their dependency on host cells for reproduction suggests they are unlikely to have been the first, as there would need to be living cells in existence for viruses to propagate.
Body's Defense Mechanisms
The immune system's white blood cells fight infections.
Physical barriers such as skin and mucous membranes prevent entry.
Chemical secretions like stomach acid destroy pathogens.
Recurring Symptoms of Viral Infections
Certain viral infections show symptoms after a period of dormancy because some viruses can remain latent within the host cells and reactivate under specific conditions, often when the immune system is compromised.