**8. Microbial carcinogenesis**

Viruses that cause tumors are called as oncogenic viruses.

Many viruses have been proven to be oncogenic in animals, but only a few have been associated with human cancer.

Classification (Flowchart): They are mainly classified depending on the genetic material into:

1.Oncogenic RNA viruses, and

2.Oncogenic DNA viruses.

Mechanism: It is similar to that of HBV and HCV-induced hepatocellular carcinoma (**Table 9**).

Bacteria:- *Helicobacter pylori*:

Diseases caused by *H. pylori* are (1) peptic ulcers, (2) gastric adenocarcinomas, and (3) gastric lymphomas, gastric adenocarcinomas.

• Chronic inflammation: *H. pylori* causes chronic inflammation (chronic gastritis) - > followed by gastric atrophy - > intestinal metaplasia -> dysplasia - > cancer,

#### *Neoplasia DOI: http://dx.doi.org/10.5772/intechopen.109512*

• Genes: *H. pylori* causing gastric adenocarcinoma contains cytotoxin-associated A (CagA) gene that can penetrate into gastric epithelial cells - > initiation of signals - > unregulated growth factor stimulation.

Gastric lymphoma:

*H. pylori* produces lymphoma of B-cell origin, which are called as lymphomas of mucosa-associated lymphoid tissue or maltomas. *H. pylori* is associated with translocation in MALT lymphomas.

Fungi:

Fungi may cause cancer by producing toxic substances (mycotoxins). Aflatoxin B, produced by Aspergillus flavus is a potent carcinogen responsible for hepatocellular carcinoma.

Parasites:

Two parasites that can cause tumors are:



#### **Table 9.**

*Various viruses implicated in human tumors and associated lesions.*
