Some infections may be contracted even if the body has antibodies, and the reasons for this may be as follows: 1. The pathogen has mutated and can evade the body’s specific immunity even though there are antibodies, such as the new coronavirus, and many people may be infected even after vaccination. 2. For example, influenza viruses are highly susceptible to mutation, and most of the influenza vaccinations are directed against a few fixed subtypes. However, there are many subtypes of influenza viruses, and they often mutate, so they cannot cover all subtypes, which may lead to reinfection caused by other types of influenza. 3. It is possible that the antibody titer has decreased and the protection is insufficient, such as the number of infected viruses is large, virulent and invasive, and the antibody titer is insufficient to form protection, which may lead to infection. 4. may lead to reinfection.