Which of the following diseases can be treated with antivirals?

Antivirals are specifically designed to target viral infections, making them effective in treating diseases caused by viruses. Cold sores, which are caused by the herpes simplex virus (HSV), fall into this category and can be effectively treated with antiviral medications such as acyclovir or valacyclovir. These medications work by inhibiting the replication of the virus, thereby reducing the severity and duration of outbreaks.

In contrast, the other conditions listed are caused by bacteria or different pathogens. Pneumococcal pneumonia is caused by the bacterium Streptococcus pneumoniae and is typically treated with antibiotics rather than antivirals. Strep throat, caused by group A streptococcus, also requires antibiotic treatment. Syphilis is a sexually transmitted infection caused by the bacterium Treponema pallidum and is treated with antibiotics, specifically penicillin. Thus, cold sores are the only condition among those listed that is treated with antiviral medications.