What type of infection is caused by microorganisms that multiply in a host?

The correct answer is a viral infection. Viral infections occur when viruses invade a host's cells and use the host's cellular machinery to replicate and multiply. Viruses are obligate parasites, meaning they cannot reproduce outside of a living host, which distinguishes them from other types of microorganisms.

In the context of infection, once inside a host, the virus can cause various illness symptoms by hijacking the host's biological processes. This characteristic of depending on a host for replication is fundamental to viral infections.

Although fungal, bacterial, and parasitic infections also involve microorganisms multiplying within a host, their mechanisms, and the types of organisms involved differ from those of viral infections. For example, bacteria can replicate independently outside a host, while fungi have their own methods of reproduction and growth. Therefore, focusing on the defining feature of replication involving a host highlights why viral infections are the correct choice for this question.